Add Datadog/statsd with simple client emitting startup

[forge33]

Relates to: #1672

Description

This PR adds a simple setup around DataDog/StatsD by adding two new arguments when calling gh-ost:

• --statsd-addr
• --statsd-tags

This will start emitting the gh_ost.startup metric to statsd.

This is being upstreamed from Shopify's fork — original PR: Shopify#30

🎩

Without tags

With some tags

--statsd-addr=127.0.0.1:9125
--statsd-tags=table:foo,identifier:bar,application:baz,database:qux

In case this PR introduced Go code changes:

• contributed code is using same conventions as original code
• script/cibuild returns with no formatting errors, build errors or unit test errors.