Add a statsd client module

prometheus-statsd-exporter uses the datadog format for its tags, so base our
client on their Python code, with the following modifications:

• Removed python < 3.5 compat code
• trimmed the imports down to be a single module
• move to time.monotonic() instead of time.time()
• adjust some options:
  • drop unix socket connection option
  • add environment variable support for setting the statsd host and port (pulled the idea from the main python statsd module)
  • only send timer metrics in milliseconds (that's what prometheus-statsd-exporter expects)
  • drop DataDog-specific metric types (that are unsupported in prometheus-statsd-exporter)
• made the tags a dict instead of a list (prometheus-statsd-exporter only supports tags with a value, mirroring prometheus)
• improve unit test coverage
• documentation cleanup