feat(instrumentation-pg): initial implementation of DB metrics

[maryliag]

Which problem is this PR solving?

• Part Of Prototype for Semantic Conventions for Database Metrics #2348

Short description of the changes

• This PR implements part of the Semantic Conventions for Database Metrics. I will break down the implementation on different PRs to facilitate the review process.
• This PR focus on the metrics:
  • db.client.connection.count
  • db.client.connection.pending_requests

[codecov]

Codecov Report

Attention: Patch coverage is 96.87500% with 1 line in your changes missing coverage. Please review.

Project coverage is 90.70%. Comparing base (dfb2dff) to head (c095d5a).
Report is 236 commits behind head on main.

Files with missing lines	Patch %	Lines
...elemetry-instrumentation-pg/src/instrumentation.ts	92.85%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2349      +/-   ##
==========================================
- Coverage   90.97%   90.70%   -0.27%     
==========================================
  Files         146      156      +10     
  Lines        7492     7664     +172     
  Branches     1502     1578      +76     
==========================================
+ Hits         6816     6952     +136     
- Misses        676      712      +36     

Files with missing lines	Coverage Δ
...node/opentelemetry-instrumentation-pg/src/utils.ts	97.61% <100.00%> (+0.64%)	⬆️
...elemetry-instrumentation-pg/src/instrumentation.ts	88.67% <92.85%> (+0.70%)	⬆️

... and 74 files with indirect coverage changes

[maryliag]

@pichlermarc friendly ping if this can be merged