Add support for Datadog Service Checks

[jeffutter]

These are implemented as last_value metrics with report_as: :service_check. It does not support the "host" or "message" property, since TelemetryMetrics doesn't really seem to have a good way to express them.

Fixes #72

[arkgil]

It does not support the "host" or "message" property

@jeffutter are those just tags that need to be sent with the service check event?

[jeffutter]

It does not support the "host" or "message" property

@jeffutter are those just tags that need to be sent with the service check event?

I don't believe they are just tags. They are additional custom fields in the statsd payload. You can see the reference here: https://docs.datadoghq.com/developers/dogstatsd/datagram_shell/?tab=servicechecks

E.x. _sc|<NAME>|<STATUS>|d:<TIMESTAMP>|h:<HOSTNAME>|#<TAG_KEY_1>:<TAG_VALUE_1>,<TAG_2>|m:<SERVICE_CHECK_MESSAGE> the m field isn't a normal tag.