trace/sql: Wrap and return standard library sql.DB

[nstratos]

This change adds two functions to package trace/sql. One that wraps the
standard library sql.DB, which provides support for libraries like
sqlmock and another that provides access to the wrapped sql.DB which is
useful for SQL code that does not require tracing.

Closes #179.

Signed-off-by: Stratos Neiros s.neiros@thebeat.co

[codecov]

Codecov Report

Merging #178 into master will increase coverage by 4.35%.
The diff coverage is 95.28%.

@@            Coverage Diff             @@
##           master     #178      +/-   ##
==========================================
+ Coverage   80.04%   84.40%   +4.35%     
==========================================
  Files          47       51       +4     
  Lines        2551     3046     +495     
==========================================
+ Hits         2042     2571     +529     
+ Misses        447      397      -50     
- Partials       62       78      +16     

Impacted Files	Coverage Δ
async/amqp/amqp.go	45.58% <0.00%> (ø)
trace/sqs/message.go	92.50% <92.50%> (ø)
async/kafka/kafka.go	92.59% <92.85%> (-0.10%)	⬇️
component/grpc/observability.go	93.25% <93.25%> (ø)
trace/kafka/kafka.go	84.12% <93.33%> (+2.87%)	⬆️
trace/sqs/publisher.go	93.33% <93.33%> (ø)
component/grpc/component.go	94.11% <94.11%> (ø)
sync/http/component.go	96.89% <94.87%> (-1.48%)	⬇️
cmd/patron/main.go	45.31% <95.00%> (ø)
service.go	92.17% <95.28%> (+13.49%)	⬆️
... and 19 more

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