[service] Move batchprocessor metrics to normal level and update level guidelines

[jade-guiton-dd]

Description

This PR:

• requires "level: normal" before outputting batch processor metrics (in addition to one specific metric which was already restricted to "level: detailed")
• clarifies wording in the telemetry level guidelines and documentation, and adds said guidelines to the requirements for stable components.

Some rationale for these changes can be found in the tracking issue and this comment.

Link to tracking issue

Resolves #7890

To be discussed

Should we add a feature gate for this, in case a user relies on "level: basic" outputting batch processor metrics? This feels like a niche use case, so considering the "alpha" stability level of these metrics, I don't think it's really necessary.

Considering batch processor metrics had already been switched to "normal" once (#9767), but were turned back to basic at some later point (not sure when), we might also want to add tests to avoid further regressions (especially as the handling of telemetry levels is bound to change further with #11754).

[codecov]

Codecov Report

Attention: Patch coverage is 83.33333% with 1 line in your changes missing coverage. Please review.

Project coverage is 92.02%. Comparing base (8099e51) to head (5fa6cec).
Report is 44 commits behind head on main.

Files with missing lines	Patch %	Lines
service/service.go	83.33%	0 Missing and 1 partial ⚠️

❌ Your patch status has failed because the patch coverage (83.33%) is below the target coverage (95.00%). You can increase the patch coverage or adjust the target coverage.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #12525      +/-   ##
==========================================
- Coverage   92.15%   92.02%   -0.14%     
==========================================
  Files         465      470       +5     
  Lines       25183    25376     +193     
==========================================
+ Hits        23208    23351     +143     
- Misses       1576     1617      +41     
- Partials      399      408       +9     

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[dmitryax]

LGTM. Thanks, @jade-guiton-dd!