Introduce Prometheus metrics

[hackerwins]

What type of PR is this?

Uncomment only one /kind <> line, hit enter to put that in a new line, and remove leading whitespace from that line:

/kind api-change
/kind bug
/kind cleanup
/kind deprecation
/kind design
/kind documentation
/kind failing-test

/kind feature

What this PR does / why we need it:

Yorkie is getting more complex as features are added. So, we need to monitor the status of Agent.
This PR introduces prometheus and shows you how to add application specific metrics.

Which issue(s) this PR fixes:

Fixes #76

Special notes for your reviewer:

Does this PR introduce a user-facing change?:

Additional documentation:

- [Instrumenting a Go application for prometheus]: https://prometheus.io/docs/guides/go-application/

[codecov]

Codecov Report

Merging #80 into master will decrease coverage by 0.20%.
The diff coverage is 34.61%.

@@            Coverage Diff             @@
##           master      #80      +/-   ##
==========================================
- Coverage   55.55%   55.35%   -0.21%     
==========================================
  Files          23       23              
  Lines        2709     2728      +19     
==========================================
+ Hits         1505     1510       +5     
- Misses       1107     1123      +16     
+ Partials       97       95       -2     

Impacted Files	Coverage Δ
yorkie/config.go	13.88% <0.00%> (-1.27%)	⬇️
yorkie/yorkie.go	0.00% <0.00%> (ø)
yorkie/rpc/server.go	44.72% <100.00%> (+1.68%)	⬆️
pkg/llrb/llrb.go	76.55% <0.00%> (-1.38%)	⬇️

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