Add a debug logging interface

[vprithvi]

Relates to #501

• Add a debug log interface so that the client may log more details when the supplied logger supports debug logging and is configured to log debug logs.
• Add a debug log adapter that converts a standard logger to a debug logger if it satisfies the new interface. If it doesn't, debug logging is disabled.

[codecov]

Codecov Report

Merging #503 into master will increase coverage by 0.06%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #503      +/-   ##
==========================================
+ Coverage   88.43%   88.50%   +0.06%     
==========================================
  Files          59       59              
  Lines        3581     3600      +19     
==========================================
+ Hits         3167     3186      +19     
  Misses        303      303              
  Partials      111      111              

Impacted Files	Coverage Δ
sampler_remote.go	82.35% <ø> (ø)
tracer.go	95.88% <ø> (ø)
log/logger.go	100.00% <100.00%> (ø)
log/zap/logger.go	100.00% <100.00%> (ø)
reporter_options.go	100.00% <100.00%> (ø)
sampler_remote_options.go	100.00% <100.00%> (ø)
tracer_options.go	79.10% <100.00%> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update ef8f5ed...95544e2. Read the comment docs.