feat: add example for postgres plugin

[xiao-lix]

Which problem is this PR solving?

• Requires feat(plugin): pg-pool plugin implementation #501
• Closes examples: provide an example for postgres plugin #491

Short description of the changes

• Add an express app example using postgres plugin

[codecov-io]

Codecov Report

Merging #542 into master will decrease coverage by 1.75%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #542      +/-   ##
==========================================
- Coverage   90.54%   88.79%   -1.76%     
==========================================
  Files         196      195       -1     
  Lines        9680     9798     +118     
  Branches      882      900      +18     
==========================================
- Hits         8765     8700      -65     
- Misses        915     1098     +183

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%)	⬇️
...kages/opentelemetry-metrics/test/mocks/Exporter.ts	66.66% <0%> (-33.34%)	⬇️
...ntelemetry-core/test/trace/NoRecordingSpan.test.ts	71.42% <0%> (-28.58%)	⬇️
... and 24 more

[mayurkale22]

lgtm, added a few minor comments

[mayurkale22]

@open-telemetry/javascript-approvers Please review and approve if looks good.

[OlivierAlbertini]

LGTM.

[mayurkale22]

The dependent PR (#501) is already merged, merging this one now. Thanks for the work!