Apm: Add global labels to spans

[felixbarny]

Currently, global labels are only added to transactions, metrics, and errors. There are several requests for enabling global labels for spans too.

The latest one: https://discuss.elastic.co/t/labels-are-getting-added-to-only-transaction-event-not-to-span-event/201069/

Some other use cases:

• Send all events with a particular global label (for example department) to a specific index. By doing that it's possible to set up different retention/ILM policies based on the global label.
• Use the global label for access control (see also https://github.com/elastic/apm/issues/99)

[felixbarny]

The environment will be added to all spans as of 7.4: https://github.com/elastic/apm-server/issues/2471

IMHO, it would be consistent to do the same for global labels. Does someone have objections in terms of disk usage? I don't expect that users would add lots and lots of global labels. And if they add them, they generally want to be able to filter by those labels.

[Qard]

Personally, I'm all for _consistency_. The less differences between transactions and spans the better.

[nehaduggal]

Filtering spans by labels has come up in multiple user conversations. In most cases its users wanting to add just a couple of labels(at the most)to track specific end to end user actions.

[felixbarny]

Based on the comments here and in today's meeting, it seems like there's agreement that this should be done. @graphaelli could you create an APM Server issue and put it in a milestone?

[ramya-govind]

When can I expect this change? In which release it will be updated?

[graphaelli]

Issue created but milestone decision is still pending, feel free to follow the linked issue.

[jalvz]

This will go out in 7.6.

https://github.com/elastic/apm-server/pull/2806

[jalvz]

Server support has been merged. If agents do not need a new API or anything, this issue can be closed.

[axw]

AFAIK, they do not. We can reopen if needed.