Make registration of callback for async metric always optional

[bogdandrutu]

This PR also solves one more issues with the new interface, which is the ability to record multiple observations for a single async metric using one callback.

[codecov]

Codecov Report

Attention: Patch coverage is 67.01847% with 125 lines in your changes missing coverage. Please review.

Project coverage is 91.28%. Comparing base (d977da4) to head (53e231f).
Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
service/internal/metadata/generated_telemetry.go	54.23%	42 Missing and 12 partials ⚠️
...terhelper/internal/metadata/generated_telemetry.go	56.25%	25 Missing and 3 partials ⚠️
...ereceiver/internal/metadata/generated_telemetry.go	74.41%	9 Missing and 2 partials ⚠️
...processor/internal/metadata/generated_telemetry.go	65.38%	7 Missing and 2 partials ⚠️
...ervice/internal/proctelemetry/process_telemetry.go	84.37%	4 Missing and 1 partial ⚠️
cmd/mdatagen/internal/samplereceiver/factory.go	82.35%	2 Missing and 1 partial ⚠️
processor/batchprocessor/metrics.go	62.50%	2 Missing and 1 partial ⚠️
...processor/internal/metadata/generated_telemetry.go	50.00%	2 Missing and 1 partial ⚠️
...sorhelper/internal/metadata/generated_telemetry.go	50.00%	2 Missing and 1 partial ⚠️
...verhelper/internal/metadata/generated_telemetry.go	50.00%	2 Missing and 1 partial ⚠️
... and 1 more

❌ Your patch status has failed because the patch coverage (67.01%) 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   #12204      +/-   ##
==========================================
- Coverage   91.84%   91.28%   -0.57%     
==========================================
  Files         465      465              
  Lines       25325    25608     +283     
==========================================
+ Hits        23261    23376     +115     
- Misses       1675     1819     +144     
- Partials      389      413      +24     

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[dmitryax]

LGTM