chore: add instruction for pg-pool plugin

[xiao-lix]

Which problem is this PR solving?

• Closes chore(docs:postgres): add usage instructions #539

Short description of the changes

• Add usage instruction for pg-pool plugin.

[obecny]

@xiao-lix I think you should create an example with "new approach" (registry, getTracer, init etc.)

[codecov-io]

Codecov Report

Merging #733 into master will decrease coverage by 1.51%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #733      +/-   ##
==========================================
- Coverage   92.59%   91.07%   -1.52%     
==========================================
  Files         229      225       -4     
  Lines       10306    10456     +150     
  Branches      933      966      +33     
==========================================
- Hits         9543     9523      -20     
- Misses        763      933     +170

Impacted Files	Coverage Δ
packages/opentelemetry-core/test/utils/url.test.ts	50% <0%> (-50%)	⬇️
...pentelemetry-core/test/internal/validators.test.ts	50% <0%> (-50%)	⬇️
...elemetry-core/test/trace/spancontext-utils.test.ts	55.55% <0%> (-44.45%)	⬇️
...lemetry-core/test/trace/ProbabilitySampler.test.ts	56.52% <0%> (-43.48%)	⬇️
...s/opentelemetry-core/test/trace/NoopTracer.test.ts	60% <0%> (-40%)	⬇️
...s/opentelemetry-core/test/context/B3Format.test.ts	63.39% <0%> (-36.61%)	⬇️
...ges/opentelemetry-core/test/trace/NoopSpan.test.ts	63.63% <0%> (-36.37%)	⬇️
...s/opentelemetry-core/test/trace/tracestate.test.ts	65.06% <0%> (-34.94%)	⬇️
...ntelemetry-core/test/trace/NoRecordingSpan.test.ts	71.42% <0%> (-28.58%)	⬇️
...ackages/opentelemetry-core/src/platform/node/id.ts	71.42% <0%> (-28.58%)	⬇️
... and 49 more

[obecny]

lgtm, just added few comments with regards to copy

[mayurkale22]

Lgtm