Support statsd for vitess

[5antelope]

This tries to address #7064 and add very basic initial supports for statsd (we might need to tweak it a bit more later)

This PR mainly does 2 things:

1. Make statsd available as a plugin for VTTablet and VTGate
2. Export stats.Timings as histogram for now until we have better solution to store and export metrics directly as statsd.timing

[5antelope]

Not sure if this build failure is related to my change https://github.com/vitessio/vitess/pull/7072/checks?check_run_id=1449541891 @sougou do you happen to know why this might fail on top of your head? Thanks!

[deepthi]

Not sure if this build failure is related to my change https://github.com/vitessio/vitess/pull/7072/checks?check_run_id=1449541891 @sougou do you happen to know why this might fail on top of your head? Thanks!

We have seen this test fail before (randomly). I don't think it is related to your changes.

[5antelope]

Good to know, thanks for confirming!

@deepthi / @sougou would any of you happen to be available to take a look at this change and see if it makes sense to you? Thanks!

[sougou]

Nice work. It's good to see that the feature is disabled unless stats_address is specified.