Deprecate setting prefix on metric calls

[wvanbergen]

• Deprecate StatsD.measure(..., as_dict: true). AFAICS this keyword argument was mostly meant for internal use, but is part of a public method signature. You should use StatsD.distribution directly instead.
• Deprecate StatsD.metric(..., prefix: "foo"). You can simply incorporate the prefix in the metric name. If you want to override the global prefix, set no_prefix to true and incorporate your desired prefix in the name directly.

I've modified strict mode to blow up when these keyword arguments are used. Testing that on the Shopify codebase now.

[casperisfine]

Ultimately different prefix should be different clients.

That or go redis-namespace style.

[wvanbergen]

Ultimately different prefix should be different clients.

That's the plan with the new client:

prefixed_client = StatsD.client.with_prefix('foo')
prefixed_client.increment('bar')