Add instrumentation and expose measurement in prometheus format

[StevenYCChou]

Add interface inside api_server, so we expose instrumentation metrics later on (currently working on it).

Some thoughts:

1. I added a private function MetadataApiServer::SerializeMetricsToPrometheusTextFormat() so we can test the functionality without considering http_connection, which isn't tested in current test suites.
2. I only built prometheus-cpp core library with gzip compression disabled for 2 reason: 1) gzip is not used in our http server, 2) gzip compression requires system library unless we statically compile and link with gzip library.

[StevenYCChou]

I noticed we use a lot of space indentation in Makefile, though I think only tab indentation should be used in Makefile. Will use separate PR to change that indentation.

[StevenYCChou]

@g-easy can you take a brief look on this PR? I want to get your feedback on

• integrating opencensus-cpp library with building system (CMake and make)
• code structure on using opencensus-cpp
• application testing strategy with opencensus-cpp

[StevenYCChou]

change from RFC to normal one, because this one has tests included. high level review is still welcome first so we can get feedback loop soon.