Allow More Custom Tags

[aterris]

Looking through the code I noticed that you have limited custom tags in b6868b1

I ran into this as I was trying to add some custom tags to some of my custom metrics. I was wondering if you could shed any light on the context behind that change?

I was thinking about doing one of the following...

1. Updating docs to reflect that
2. Remove the constraint entirely
3. Enable setting allowed custom tags via environment variable

Before I made a PR, I wanted to understand why you made that change in the first place.

[w-vi]

Yes they are limited because datadog does not handle very well if you have loads of them. This list should actually be configurable but apparently this has never happened. I this having the list defined by environment variable is a good approach so if you have a PR ready then please go ahead.

[aterris]

Not quite ready but should be able to create one soon. Thanks!