StatsD support should be optional

[omad]

I think that StatsD support in the odc-tools applications should be optional.

It feels wrong to me to have a mandatory dependency on datadog, which in my mind is a commercial service offering that is not required required for running the ODC Tools indexing tools. It feels like an endorsement or recommendation, and I'm not comfortable with that.

The tools can operate normally by simply not reporting metrics to a statsd server.

Also, using the datadog package instead of a more generic statsd package seems somewhat unnecessary. But if it's changed to be optional I think it's okay.