[Hints Support] Add default host

[constanca-m]

What does this PR do?

Given a pod with containers, it adds co.elastic.hints.<container-name>/host: '${kubernetes.pod.ip}:<container-port>' to the annotations for each container.

Why is it important?

The user no longer needs to specify the host for the hints, which until now was a mandatory step.

Checklist

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have made corresponding change to the default configuration files
• I have added tests that prove my fix is effective or that my feature works
• I have added an entry in ./changelog/fragments using the changelog tool
• I have added an integration test or an E2E test

How to test this PR locally

1. Clone this branch.
2. Follow the steps in the README file to create the EA and add it to Kubernetes.
3. Deploy a pod with at least one container and the required annotation co.elastic.hints/package and check if the results can be seen in ES.

Related issues

• Closes Remove the need to define host in hints autodiscovery by using proper default values #1453
• Relates to [Meta] Improving Hints Autodiscovery with Elastic Agent #3063

Results

Context: We have a nginx container with two pods: one in port 80, nginx-1, and another in port 81, nginx-2.

The manifest file for that would be something like this.

apiVersion: v1
kind: Pod
metadata:
  name: nginx
  annotations:
    co.elastic.hints/package: nginx
spec:
  volumes:
    - name: config
      configMap:
        name: nginx-conf
  containers:
    - name: nginx-1
      image: nginx
      ports:
        - containerPort: 80
    - name: nginx-2
      image: nginx
      ports:
        - containerPort: 81
      volumeMounts:
        - name: config
          mountPath: /etc/nginx/conf.d

Situation 1: Two pods and no annotation for host

If we have annotations only for the package:

  annotations:
    co.elastic.hints/package: nginx

Then the defaults for the host will be used.

Inspecting the output of the inspect command (elastic-agent inspect -v --variables --variables-wait 2s) results in this:

...
- data_stream.namespace: default
  id: <...>.nginx-1 <--------------------------- Our container nginx-1
  name: nginx/metrics-nginx
  ...
  streams:
    - data_stream:
        dataset: nginx.stubstatus
        type: metrics
      hosts:
        - 10.244.0.6:80 <---------------------------- Host of nginx-1
...
- data_stream.namespace: default
  id: <...>.nginx-2  <--------------------------- Our container nginx-2
  name: nginx/metrics-nginx
  ...
  streams:
    - data_stream:
        dataset: nginx.stubstatus
        type: metrics
      hosts:
        - 10.244.0.6:81 <---------------------------- Host of nginx-2
...

Checking the unique count of the containers has this output:

Situation 2: Two pods and only one host annotation for one pod

The annotations would be like this:

  annotations:
    co.elastic.hints/package: nginx
    co.elastic.hints.nginx-1/host: '${kubernetes.pod.ip}:80'

The result is the same as situation 1, as the host for nginx-2 will be the default ('${kubernetes.pod.ip}: + port).

Situation 3: Two pods and one top level annotation

The annotations would be like this:

  annotations:
    co.elastic.hints/package: nginx
    co.elastic.hints/host: '${kubernetes.pod.ip}:82'

In this case, we are using a port that is not even possible (there is no nginx pod with port 82), to make sure the host works.

The output is the exact same as situation 1. The following happened in our code:

1. Checked the mappings for each container.
2. There was no default host defined for any of the containers.
3. Then add the default host for each container.

[mergify]

This pull request does not have a backport label. Could you fix it @constanca-m? 🙏
To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

• backport-v./d./d./d is the label to automatically backport to the 8./d branch. /d is the digit

NOTE: backport-skip has been added to this pull request.

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• Start Time: 2023-10-10T13:01:28.018+0000

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[constanca-m]

/test

[gizas]

Do we need the stream here? If not shall we remove it from all tests?

[constanca-m]

We don't need it, I just wanted to make sure that adding the default host was not overwriting any of the hints.

[gizas]

@constanca-m you should create a fragment to pass the CI failure.
As per https://github.com/elastic/elastic-agent#changelog

elastic-agent-changelog-tool new <some title>

[ChrsMark]

I think in the description the term "pod" is mixed with the term "container". For example co.elastic.hints.<pod-name>/host: '${kubernetes.pod.ip}:<container-port>' should be co.elastic.hints.<container-name>/host: '${kubernetes.pod.ip}:<container-port>' right? Same in the manual testing examples' tittles.

[ChrsMark]

Nice addition!

I have left some comments to consider as improvements. I have not tested the patch manually but maybe it would worth it to introduce some manual testing phase for such PRs (since it seems that e2e is not used any more?).

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