Add support for more environment variables

[delner]

This pull request adds global tags for:

• DD_TAGS (acts as base set of global tags, overridden by other tags)
• DD_ENV --> env:<env name>
• DD_SERVICE --> service:<service name>
• DD_VERSION --> version:<version string>

Precedence in order from most to least preferred is manually defined global tags (via #initialize), DD_ENV/DD_SERVICE/DD_VERSION, then finally DD_TAGS.

[remeh]

LGTM 👍
@kbogtob will probably want to give it a look before merging though.

[delner]

@remeh Okay, there's one more question I had... if we set these global tags (say env:staging), and the user passes statsd.count('hits', 1, tags: ['env:staging']), will this produce duplicate env:staging tags? If so would we want to remove this kind of duplication?

Just thinking of the trade-off between saving on packet size vs saving on memory/CPU it would take to de-dup this.

[kbogtob]

Thanks @delner for your contribution!
I've just fixed some nits to homogenise the code with the existing one and added some tests.

It's true that we don't deduplicate the tags and I think we should be able to override specific tags in this order: ENV < Global tags < Call tags. I will discuss it internally and create an issue.

PS: Thanks for ClimateControl, didn't know that gem before :). It's nice.

[delner]

@kbogtob Changes seem fine to me. Also just added a minor fix for a warning that was blowing up our test suite (when we'd close Statsd without using it.)

[kbogtob]

LGTM. Just left a small nit.