SQL comment propagation for mysql2

[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 #2324 (e64d994) into master (277f4bb) will increase coverage by 0.00%.
The diff coverage is 99.51%.

@@           Coverage Diff            @@
##           master    #2324    +/-   ##
========================================
  Coverage   98.30%   98.30%            
========================================
  Files        1092     1100     +8     
  Lines       58047    58242   +195     
========================================
+ Hits        57062    57256   +194     
- Misses        985      986     +1     

Impacted Files	Coverage Δ
...tadog/tracing/contrib/sinatra/tracer_middleware.rb	94.73% <0.00%> (ø)
...g/tracing/contrib/mysql2/configuration/settings.rb	100.00% <100.00%> (ø)
.../datadog/tracing/contrib/mysql2/instrumentation.rb	100.00% <100.00%> (ø)
...datadog/tracing/contrib/propagation/sql_comment.rb	100.00% <100.00%> (ø)
...tracing/contrib/propagation/sql_comment/comment.rb	100.00% <100.00%> (ø)
...dog/tracing/contrib/propagation/sql_comment/ext.rb	100.00% <100.00%> (ø)
...og/tracing/contrib/propagation/sql_comment/mode.rb	100.00% <100.00%> (ø)
...pec/datadog/tracing/contrib/mysql2/patcher_spec.rb	100.00% <100.00%> (ø)
...ng/contrib/propagation/sql_comment/comment_spec.rb	100.00% <100.00%> (ø)
...acing/contrib/propagation/sql_comment/mode_spec.rb	100.00% <100.00%> (ø)
... and 5 more

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[marcotc]

Anything keeping this PR in draft state?

[TonyCTHsu]

@marcotc I am still working on the documentation for this 😄

[marcotc]

🔥