Fix issue with unnecessary config reload in static mode

[ptodev]

PR Description

When a config reload is triggered, the config shouldn't be reloaded if it hasn't changed. Unfortunately, there was a bug which prevented this for the "metrics" and "logs" subsystems:

• The X-Agent-Id header isn't in the user-defined config.
• In the "logs" subsytem, the scrape_configs/static_configs section, there might be a targets: [localhost] which was added by default by Promtail because the user didn't set it explicitly

For example, when using static_mode.yaml.txt, the Agent tries interprets the new config as example_incoming.yaml.txt. This doesn't match the way it thinks the "current" config that it uses is defined(example_running.yaml.txt), so it does a config reload. This issue is not present in Flow - it's only in static mode.

There might also be similar issues that cause mismatch in configs. It's possible that I don't see all such issues, since my config file is using very few functionality. The only sure way of eliminating these issues that I can think of is to store the incoming config in a string, as this PR is doing.

Notes to the Reviewer

This is a prerequisite for #6971

PR Checklist

• CHANGELOG.md updated
• Documentation added
• Tests updated
• Config converters updated