module/apmgocql: add tracing observer

[axw]

Add an observer for gocql single and batch query executions, which creates spans if the context contains a transaction.

Closes #34

[codecov-io]

Codecov Report

Merging #148 into master will increase coverage by 0.55%.
The diff coverage is 97.5%.

@@            Coverage Diff             @@
##           master     #148      +/-   ##
==========================================
+ Coverage   77.43%   77.99%   +0.55%     
==========================================
  Files          65       67       +2     
  Lines        3856     3976     +120     
==========================================
+ Hits         2986     3101     +115     
- Misses        663      666       +3     
- Partials      207      209       +2

Impacted Files	Coverage Δ
internal/sqlscanner/scanner.go	88.88% <ø> (ø)
internal/sqlscanner/token.go	60% <ø> (ø)
module/apmsql/signature.go	84.7% <ø> (ø)	⬆️
module/apmgocql/observer.go	95.65% <95.65%> (ø)
module/apmgocql/signature.go	98.64% <98.64%> (ø)
tracer.go	84.34% <0%> (-0.58%)	⬇️

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