Support for using statsd.Client as an otel.MeterProvider

[imle]

Looking to get some comments and guidance on this approach before getting to far into the weeds.

After a number of attempts, I settled on implementing the MeterProvider interface as a passthrough for otel metrics to the statsd.Client. This keeps much of the pipelineing system from otel out of the way of keeping things lined up with how this library already sends metrics.

Outstanding Work:

• observable metrics, the whole reason this needs to be implemented in the statsd library
• callbacks registered against the meter
• verify that it is acceptable to send a float64 as a Counter value
  it does not appear to be acceptable
• cleanup and improve readability and repeated work

[carlosroman]

I wonder if this will break our compatibility with older versions of Golang. At the moment datadog-go compiles and works with versions all the way back to go 1.12.
This is an interesting approach to being able to reuse our Golang client to send OTel metrics. I'll discuss this internally to see what people think.