[UII] Allow to reset log level for agents >= 8.15.0 (#183434)

‼️ Should be reverted if
elastic/elastic-agent#4747 does not make
8.15.0.

## Summary

Resolves #180778 

This PR allows agent log level to be reset back to the level set on its
policy (or if not set, simply the default agent level, see
elastic/elastic-agent#3090).

To achieve this, this PR:
- Allows `null` to be passed for the log level settings action, i.e.:

```
POST kbn:/api/fleet/agents/<AGENT_ID>/actions
{"action":{"type":"SETTINGS","data":{"log_level": null}}}
```
- Enables the agent policy log level setting implemented in
#180607
- Always show `Apply changes` on the agent details > Logs tab
- For agents >= 8.15.0, always show `Reset to policy` on the agent
details > Logs tab
- Ensures both buttons are disabled if user does not have access to
write to agents

<img width="1254" alt="image"
src="https://github.com/elastic/kibana/assets/1965714/bcdf763e-2053-4071-9aa8-8bcb57b8fee1">

<img width="1267" alt="image"
src="https://github.com/elastic/kibana/assets/1965714/182ac54d-d5ad-435f-9376-70bb24f288f9">

### Caveats
1. The reported agent log level is not accurate if agent is using the
level from its policy and does not have a log level set on its own level
(elastic/elastic-agent#4747), so the initial
selection on the agent log level could be wrong
2. We have no way to tell where the log level came from
(elastic/elastic-agent#4748), so that's why
`Apply changes` and `Reset to policy` are always shown

### Testing
Use the latest `8.15.0-SNAPSHOT` for agents or fleet server to test this
change

### Checklist

Delete any items that are not applicable to this PR.

- [x] Any text added follows [EUI's writing
guidelines](https://elastic.github.io/eui/#/guidelines/writing), uses
sentence case text and includes [i18n
support](https://github.com/elastic/kibana/blob/main/packages/kbn-i18n/README.md)
- [x] [Unit or functional
tests](https://www.elastic.co/guide/en/kibana/master/development-tests.html)
were updated or added to match the most common scenarios

x-pack/plugins/fleet/common/constants/agent_policy.ts

@@ -39,17 +39,10 @@ export const DEFAULT_MAX_AGENT_POLICIES_WITH_INACTIVITY_TIMEOUT = 750;
 export const AGENTLESS_POLICY_ID = 'agentless'; // the policy id defined here: https://github.com/elastic/project-controller/blob/main/internal/project/security/security_kibana_config.go#L86
 
 export const AGENT_LOG_LEVELS = {
-  ERROR: 'error',
-  WARNING: 'warning',
-  INFO: 'info',
-  DEBUG: 'debug',
+  info: 'info',
+  debug: 'debug',
+  warning: 'warning',
+  error: 'error',
 };
 
-export const DEFAULT_LOG_LEVEL = AGENT_LOG_LEVELS.INFO;
-
-export const agentLoggingLevels = {
-  Info: 'info',
-  Debug: 'debug',
-  Warning: 'warning',
-  Error: 'error',
-} as const;
+export const DEFAULT_LOG_LEVEL = AGENT_LOG_LEVELS.info;

x-pack/plugins/fleet/common/openapi/bundled.json

@@ -7294,6 +7294,7 @@
                 "properties": {
                   "log_level": {
                     "type": "string",
+                    "nullable": true,
                     "enum": [
                       "debug",
                       "info",

x-pack/plugins/fleet/common/openapi/bundled.yaml

@@ -4674,6 +4674,7 @@ components:
               properties:
                 log_level:
                   type: string
+                  nullable: true
                   enum:
                     - debug
                     - info

x-pack/plugins/fleet/common/openapi/components/schemas/agent_action.yaml

@@ -19,6 +19,7 @@ oneOf:
         properties:
           log_level:
             type: string
+            nullable: true
             enum:
               - debug
               - info

x-pack/plugins/fleet/common/settings/agent_policy_settings.ts → x-pack/plugins/fleet/common/settings/agent_policy_settings.tsx

@@ -4,11 +4,14 @@
  * 2.0; you may not use this file except in compliance with the Elastic License
  * 2.0.
  */
-
+import React from 'react';
 import { i18n } from '@kbn/i18n';
+import { FormattedMessage } from '@kbn/i18n-react';
+import { EuiCode } from '@elastic/eui';
+
 import { z } from 'zod';
 
-import { agentLoggingLevels } from '../constants';
+import { AGENT_LOG_LEVELS, DEFAULT_LOG_LEVEL } from '../constants';
 
 import type { SettingsConfig } from './types';
 
@@ -130,20 +133,19 @@ export const AGENT_POLICY_ADVANCED_SETTINGS: SettingsConfig[] = [
   },
   {
     name: 'agent.logging.level',
-    hidden: true,
     title: i18n.translate('xpack.fleet.settings.agentPolicyAdvanced.agentLoggingLevelTitle', {
       defaultMessage: 'Agent logging level',
     }),
-    description: i18n.translate(
-      'xpack.fleet.settings.agentPolicyAdvanced.agentLoggingLevelDescription',
-      {
-        defaultMessage:
-          'Sets the log level for all the agents on the policy. The default log level is "info".',
-      }
+    description: (
+      <FormattedMessage
+        id="xpack.fleet.settings.agentPolicyAdvanced.agentLoggingLevelDescription"
+        defaultMessage="Sets the log level for all the agents on the policy. The default log level is {level}."
+        values={{ level: <EuiCode>{DEFAULT_LOG_LEVEL}</EuiCode> }}
+      />
     ),
     api_field: {
       name: 'agent_logging_level',
     },
-    schema: z.nativeEnum(agentLoggingLevels).default(agentLoggingLevels.Info),
+    schema: z.nativeEnum(AGENT_LOG_LEVELS).default(DEFAULT_LOG_LEVEL),
   },
 ];

x-pack/plugins/fleet/public/applications/fleet/sections/agents/agent_details_page/components/agent_logs/agent_logs.test.tsx

@@ -65,11 +65,15 @@ describe('AgentLogsUI', () => {
       },
     } as any);
   });
-  const renderComponent = () => {
+  const renderComponent = (
+    opts = {
+      agentVersion: '8.11.0',
+    }
+  ) => {
     const renderer = createFleetTestRendererMock();
     const agent = {
       id: 'agent1',
-      local_metadata: { elastic: { agent: { version: '8.11' } } },
+      local_metadata: { elastic: { agent: { version: opts.agentVersion, log_level: 'debug' } } },
     } as any;
     const state = {
       datasets: ['elastic_agent'],
@@ -125,4 +129,35 @@ describe('AgentLogsUI', () => {
       `http://localhost:5620/app/discover#/?_g=(filters:!(),refreshInterval:(pause:!t,value:60000),time:(from:'2023-20-04T14:00:00.340Z',to:'2023-20-04T14:20:00.340Z'))&_a=(columns:!(event.dataset,message),index:'logs-*',query:(language:kuery,query:'elastic_agent.id:agent1 and (data_stream.dataset:elastic_agent) and (log.level:info or log.level:error)'))`
     );
   });
+
+  it('should show log level dropdown with correct value', () => {
+    mockStartServices();
+    const result = renderComponent();
+    const logLevelDropdown = result.getByTestId('selectAgentLogLevel');
+    expect(logLevelDropdown.getElementsByTagName('option').length).toBe(4);
+    expect(logLevelDropdown).toHaveDisplayValue('debug');
+  });
+
+  it('should always show apply log level changes button', () => {
+    mockStartServices();
+    const result = renderComponent();
+    const applyLogLevelBtn = result.getByTestId('applyLogLevelBtn');
+    expect(applyLogLevelBtn).toBeInTheDocument();
+    expect(applyLogLevelBtn).not.toHaveAttribute('disabled');
+  });
+
+  it('should hide reset log level button for agents version < 8.15.0', () => {
+    mockStartServices();
+    const result = renderComponent();
+    const resetLogLevelBtn = result.queryByTestId('resetLogLevelBtn');
+    expect(resetLogLevelBtn).not.toBeInTheDocument();
+  });
+
+  it('should show reset log level button for agents version >= 8.15.0', () => {
+    mockStartServices();
+    const result = renderComponent({ agentVersion: '8.15.0' });
+    const resetLogLevelBtn = result.getByTestId('resetLogLevelBtn');
+    expect(resetLogLevelBtn).toBeInTheDocument();
+    expect(resetLogLevelBtn).not.toHaveAttribute('disabled');
+  });
 });

x-pack/plugins/fleet/public/applications/fleet/sections/agents/agent_details_page/components/agent_logs/agent_logs.tsx

@@ -342,7 +342,10 @@ export const AgentLogsUI: React.FunctionComponent<AgentLogsProps> = memo(
           </EuiPanel>
         </EuiFlexItem>
         <EuiFlexItem grow={false}>
-          <SelectLogLevel agent={agent} />
+          <SelectLogLevel
+            agent={agent}
+            agentPolicyLogLevel={agentPolicy?.advanced_settings?.agent_logging_level}
+          />
         </EuiFlexItem>
       </WrapperFlexGroup>
     );