Suport for collector level tags

[pavolloffay]

Similar to agent tags #833
Related to removing duplicated tags in collector #1778

Add the ability to the collector to add a set of tags to incoming spans.

The agent supports JAEGER_TAGS for adding tags, however if we support the same env var in collector the same set of tags could be added twice - although it depends if we remove duplicate tags in #1778.

Use cases:

• add tenant information cc) @radekg

[yurishkuro]

if we do this, let's make sure we reuse the code from the agent

[radekg]

@yurishkuro, would the referenced commit be close to what you'd imagine such a feature to look like? It would obviously require tests. For clarity, this is not a PR yet.

[yurishkuro]

@radekg not quite, because it adds the tags unconditionally, and there's a requirement to fix that (e.g. #1788)

[radekg]

@yurishkuro I've opened the PR so the work can be tracked. I'll add tests in the next couple of days. Regarding #1788, I wonder what's the best way to proceed. Probably wait for that to be merged first and then adapt my PR?

Edit: alternative would be to have this merged with unit tests in and then adapt #1788 to take collector tags into account in a single swipe.

[yurishkuro]

Yes, you're probably right, we can merge #1854 first, and then add the deduping policy as a separate ticket.