Integrate metrics and tracing library

[ericl]

OpenCensus provides a general purpose stats and tracing API with support for many backends, including Zipkin and Prometheus.

While we likely won't use all the functionality immediately, a good first step would be to add the library to the build and port existing debug counters to use the stats::Record() API.

There have been some concerns around backend compatibility (I.e., #3664). If needed, we can add a thin wrapper layer at the point of recording metrics to allow users to intercept the record calls, but ideally this can be avoided since OpenCensus provides push and pull APIs already for reading out metrics.

TODOs:

• Integrate opencensus into building system
• Add example metrics into raylet
• support global tags auto added
• Add all metrics for raylet
• Integrate web views into our dashboard

[robertnishihara]

@pcmoritz has been doing a lot of work on the build system recently to switch to Bazel (see #2887), so if we add it to the build, let's do it as part of Bazel and not the old build system.

[jovany-wang]

@micafan cc

[edoakes]

Stale - please open new issue if still relevant