SQL comment propagation for pg

[TonyCTHsu]

What does this PR do?

Inject sql comment to enable database monitoring.

By default, it is disabled.

With service mode, our tracer propagates service context to database, which enable Datadog to build service map.

[codecov-commenter]

Codecov Report

Merging #2339 (3a565a7) into feature/dbm-sql-comment-propagation (e64d994) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@                           Coverage Diff                           @@
##           feature/dbm-sql-comment-propagation    #2339      +/-   ##
=======================================================================
- Coverage                                98.30%   98.30%   -0.01%     
=======================================================================
  Files                                     1100     1100              
  Lines                                    58242    58283      +41     
=======================================================================
+ Hits                                     57256    57295      +39     
- Misses                                     986      988       +2     

Impacted Files	Coverage Δ
...tadog/tracing/contrib/pg/configuration/settings.rb	100.00% <100.00%> (ø)
lib/datadog/tracing/contrib/pg/instrumentation.rb	100.00% <100.00%> (ø)
spec/datadog/tracing/contrib/pg/patcher_spec.rb	100.00% <100.00%> (ø)
...adog/tracing/contrib/sidekiq/client_tracer_spec.rb	97.91% <0.00%> (-2.09%)	⬇️
lib/datadog/core/diagnostics/environment_logger.rb	98.42% <0.00%> (-1.58%)	⬇️
...ng/contrib/active_support/cache/instrumentation.rb	87.58% <0.00%> (-0.09%)	⬇️
lib/datadog/core/workers/polling.rb	100.00% <0.00%> (+3.44%)	⬆️

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