in_node_exporter_metrics: collect metrics at startup(#8368)

[nokute78]

Fixes #8368

The issue #8368 is caused by flushing metrics before updating it.
This patch is to update metrics at startup to prevent flushing empty metrics.

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Testing
Before we can approve your change; please submit the following in a comment:

• [N/A] Example configuration file for the change
• Debug log output from testing the change

• Attached Valgrind output that shows no leaks or memory corruption was found

If this is a change to packaging of containers or native binaries then please confirm it works for all targets.

• [N/A] Run local packaging test showing all targets (including any new ones) build.
• [N/A] Set ok-package-test label to test for all targets (requires maintainer to do).

Documentation

• [N/A] Documentation required for this feature

Backporting

• [N/A] Backport to latest stable release.

Debug/Valgrind output

$ valgrind --leak-check=full bin/fluent-bit -i node_exporter_metrics -o stdout -f 1 
==17236== Memcheck, a memory error detector
==17236== Copyright (C) 2002-2017, and GNU GPL'd, by Julian Seward et al.
==17236== Using Valgrind-3.18.1 and LibVEX; rerun with -h for copyright info
==17236== Command: bin/fluent-bit -i node_exporter_metrics -o stdout -f 1
==17236== 
Fluent Bit v2.2.2
* Copyright (C) 2015-2024 The Fluent Bit Authors
* Fluent Bit is a CNCF sub-project under the umbrella of Fluentd
* https://fluentbit.io

____________________
< Fluent Bit v2.2.2 >
 -------------------
(snip)
[2024/01/13 09:01:00] [ info] [fluent bit] version=2.2.2, commit=6f2c553ebc, pid=17236
[2024/01/13 09:01:00] [ info] [storage] ver=1.2.0, type=memory, sync=normal, checksum=off, max_chunks_up=128
[2024/01/13 09:01:00] [ info] [cmetrics] version=0.6.6
[2024/01/13 09:01:00] [ info] [ctraces ] version=0.4.0
[2024/01/13 09:01:00] [ info] [input:node_exporter_metrics:node_exporter_metrics.0] initializing
[2024/01/13 09:01:00] [ info] [input:node_exporter_metrics:node_exporter_metrics.0] storage_strategy='memory' (memory only)
[2024/01/13 09:01:00] [ info] [input:node_exporter_metrics:node_exporter_metrics.0] path.procfs = /proc
[2024/01/13 09:01:00] [ info] [input:node_exporter_metrics:node_exporter_metrics.0] path.sysfs  = /sys
[2024/01/13 09:01:01] [ info] [input:node_exporter_metrics:node_exporter_metrics.0] thread instance initialized
[2024/01/13 09:01:01] [ info] [output:stdout:stdout.0] worker #0 started
[2024/01/13 09:01:01] [ info] [sp] stream processor started
2024-01-13T00:01:05.988566934Z node_cpu_seconds_total{cpu="0",mode="idle"} = 3534.29
2024-01-13T00:01:05.988566934Z node_cpu_seconds_total{cpu="0",mode="iowait"} = 13.41
2024-01-13T00:01:05.988566934Z node_cpu_seconds_total{cpu="0",mode="irq"} = 0
2024-01-13T00:01:05.988566934Z node_cpu_seconds_total{cpu="0",mode="nice"} = 9.1899999999999995
2024-01-13T00:01:05.988566934Z node_cpu_seconds_total{cpu="0",mode="softirq"} = 0.040000000000000001
2024-01-13T00:01:05.988566934Z node_cpu_seconds_total{cpu="0",mode="steal"} = 0
2024-01-13T00:01:05.988566934Z node_cpu_seconds_total{cpu="0",mode="system"} = 179.81999999999999
2024-01-13T00:01:05.988566934Z node_cpu_seconds_total{cpu="0",mode="user"} = 263.85000000000002
(snip)
2024-01-13T00:01:06.031529249Z node_systemd_version{version="249.11-0ubuntu3.11"} = 249.11000000000001
^C[2024/01/13 09:01:10] [engine] caught signal (SIGINT)
[2024/01/13 09:01:10] [ warn] [engine] service will shutdown in max 5 seconds
[2024/01/13 09:01:10] [ info] [engine] service has stopped (0 pending tasks)
[2024/01/13 09:01:10] [ info] [output:stdout:stdout.0] thread worker #0 stopping...
[2024/01/13 09:01:11] [ info] [output:stdout:stdout.0] thread worker #0 stopped
==17236== 
==17236== HEAP SUMMARY:
==17236==     in use at exit: 0 bytes in 0 blocks
==17236==   total heap usage: 75,311 allocs, 75,311 frees, 2,083,706,014 bytes allocated
==17236== 
==17236== All heap blocks were freed -- no leaks are possible
==17236== 
==17236== For lists of detected and suppressed errors, rerun with: -s
==17236== ERROR SUMMARY: 0 errors from 0 contexts (suppressed: 0 from 0)

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Fluent Bit is licensed under Apache 2.0, by submitting this pull request I understand that this code will be released under the terms of that license.