Add tracing

[dennisgranath]

This adds support for distributed tracing.

By default a the NoopTracer is used which doesn't do anything. I've added a OpenCensusTracer implementation which can be used as well by setting client.withTracer(new OpenCensusTracer()).

This was very much inspired by the tracing inside the folsom client: github.com/spotify/folsom

Using lighstep it would look something like this:

om

[codecov]

Codecov Report

Merging #80 (4c6403c) into master (6490b72) will increase coverage by 0.49%.
The diff coverage is 75.29%.

@@             Coverage Diff              @@
##             master      #80      +/-   ##
============================================
+ Coverage     68.83%   69.32%   +0.49%     
- Complexity      198      217      +19     
============================================
  Files            32       35       +3     
  Lines           770      841      +71     
  Branches         33       34       +1     
============================================
+ Hits            530      583      +53     
- Misses          217      233      +16     
- Partials         23       25       +2     

Impacted Files	Coverage Δ
.../com/spotify/github/v3/clients/JwtTokenIssuer.java	100.00% <ø> (ø)
...om/spotify/github/v3/clients/RepositoryClient.java	64.15% <0.00%> (-0.67%)	⬇️
...java/com/spotify/github/v3/clients/NoopTracer.java	57.14% <57.14%> (ø)
.../com/spotify/github/opencensus/OpenCensusSpan.java	60.00% <60.00%> (ø)
...va/com/spotify/github/v3/clients/GitHubClient.java	72.84% <64.70%> (-1.93%)	⬇️
...fy/github/v3/exceptions/RequestNotOkException.java	54.54% <66.66%> (+4.54%)	⬆️
...a/com/spotify/github/v3/clients/GitDataClient.java	81.48% <86.36%> (+7.19%)	⬆️
...om/spotify/github/opencensus/OpenCensusTracer.java	100.00% <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 8b28851...4c6403c. Read the comment docs.

[barbatron]

Wouldn't passing an instrumented http client to the github client accomplish the same but without extra dependencies? (sry if I'm missing smth - I only glanced through the changes)

[dennisgranath]

So this doesn't trace the actual outgoing calls towards GHE. It just starts a span whenever a request is made and ends it whenever the request is finished (future.get() etc.).

[barbatron]

💯