Add DD_ENV environment variable

[delner]

To make it easier to tag traces with the application environment, this pull request defines DD_ENV which users can set, which will automatically:

• Be set as a tag on all traces
• Be set as a tag on all metrics
• Be added as an attribute to Datadog::Correlation for logs correlation

[delner]

NOTE: Right now, this does not reconcile any conflicts between DD_ENV, Tracer#set_tags or c.tracer tags: { ... } when each define an env tag, as this reconciliation behavior still needs to be defined.

[marcotc]

How does this PR relate to this one? #878

[delner]

Hmmmmm looks duplicative in many respects; didn't realize this existed. We probably want to reconcile these differences, take the best parts of each, and close one of these.

[brettlangdon]

do we need a test case that a span started when DD_ENV is set gets an env tag?

I assume not, since we assert that DD_ENV=test => tracer.tags = {env: 'test'}, and we know that tracer.tags = {anything} => span.meta = {anything}

[brettlangdon]

interesting, I saw this as more of a:

@env = ENV[Ext::Environment::ENV_ENVIRONMENT] if ENV.key?(Ext::Environment::ENV_ENVIRONMENT)
@tags = {}

but this could work to... I'd like to have DD_ENV take precedence over other things, but I guess in this case it doesn't matter much whether you want to manage the state as a tag or a property.

e.g.

# DD_ENV='test'
tracer.set_tags(env: 'override')

# vs

# DD_ENV=test
tracer.env = 'override'

/shrug

[delner]

Yeah, per our discussion I think we will want to move this to state as you suggest above, but we will need to refactor configuration a little bit first. We're agreed that we'd like to drive towards this though.

[delner]

So after thinking/discussing, I think the plan is:

• We will move env to configuration as suggested by @brettlangdon and @marcotc via Add configuration for application environment #878 , however for the purposes of this PR, we won't implement that here and keep the current implementation as an intermediate step. After we clean up some tech debt within configuration (that's a hurdle for our intended implementation) we will revisit and make this change.
• Thus keep this PR as is, unless there are other changes we might need to make that we haven't discussed (will wait for reviews for that.)
• Keep Add configuration for application environment #878 open in the interim, as a reminder for what we'd like to do with the change outlined in bullet point number 1.

@marcotc @brettlangdon does that sound good? Please feel free to leave your reviews for the rest of the code as well. Thanks!

[marcotc]

I like these short names :)