Add opencensus tracing bridge

[dashpole]

Issue: #93

This adds an opencensus bridge for tracing. Tracing is particularly important to have a bridge for, as different libraries used in a single go binary may use either opencensus or opentelemetry. In that case, opentelemetry and opencensus spans would not properly share parent-child relationships. The same problem doesn't exist for metrics, as a single go binary can use both opencensus and opentelemetry libraries without a loss of functionality (assuming appropriate exporters are registered for each)

Using the bridge requires overriding the opencensus default tracer with the bridge.:

octrace.DefaultTracer = bridge.NewTracer(global.Tracer("mytracer"))

This will cause all calls to the OpenCensus global functions (such as StartSpan) to call the relevant OpenTelemetry function. OpenCensus spans recorded while using this bridge will not export to OpenCensus exporters anymore. Instead, they will export to the registered opentelemetry exporter(s).

Note: This adds new go.mod/go.sum so "regular" users of opentelemetry don't have to pull in opencensus imports.
Note: This uses an unreleased commit for opencensus. I can see if I can get a release cut if that is a blocker for this PR.

[codecov]

Codecov Report

Merging #1305 (863d936) into master (27aa1f6) will increase coverage by 0.0%.
The diff coverage is 80.8%.

@@          Coverage Diff           @@
##           master   #1305   +/-   ##
======================================
  Coverage    77.3%   77.3%           
======================================
  Files         122     123    +1     
  Lines        5978    6067   +89     
======================================
+ Hits         4622    4694   +72     
- Misses       1107    1120   +13     
- Partials      249     253    +4     

Impacted Files	Coverage Δ
bridge/opencensus/bridge.go	80.8% <80.8%> (ø)

[MrAlias]

Looks good. Only comments are not blocking.

I think the use of the error handler to address incompatibilities with the APIs is the correct way to proceed. Thanks for the contribution.

[krnowak]

One nit, otherwise looks good.

[MrAlias]

@dashpole the upstream move of packages has caused conflicts here. Let me know if you would like help resolving these, I'm happy to push a fix if you don't have the time.

[dashpole]

Updated!