Allow setting user_agent

[kelunik]

Is your feature request related to a problem?

user_agent is a standard transaction field, but I can't set it via the public API: https://www.elastic.co/guide/en/apm/server/current/exported-fields-apm-transaction.html#_user_agent_4

Describe the solution you'd like

Transaction transaction = ElasticApm.currentTransaction();
transaction.setUserAgent(...);

Describe alternatives you've considered

I'm not aware of any alternatives right now, except for #433, but that's still open, too.

[JonasKunz]

Hi @kelunik,

In general we are not planning on extending the public API further, because we want to go with the OpenTelemetry-API as preferred option. We of course will continue to support the public API.

However, our OpenTelemetry API bridge also does not support overriding the built-in fields yet. This is something we eventually want to support, but it is not yet on our near term road map.

The user_agent data you are referring to is generated as follows:

• The APM-agent captures the HTTP-headers (but doesn't do any special handling of user_agent there)
• The APM-server copies the value from http.request.headers.User-Agent into user_agent.original
• the APM traces ingest-pipeline takes care of parsing the value and generating the other user_agent.* fields

I can suggest you to try the following workaround for now:

• Capture your override for user_agent as a normal label via the public API
• Add a custom ingest pipeline for moving the value from your custom label into user_agent.original
• Your custom ingest pipeline will have to redo the parsing and generation of the other user_agent.* fields, but you should be able to copy that from the standard APM ingest pipeline

We also would be willing to accept a PR for adding a Transaction.setHeader() method to the public-API, though es explained earlier we would prefer to not extend it, as we'll have to continue to support it.

[kelunik]

* The APM-server copies the value from `http.request.headers.User-Agent` into `user_agent.original`

That's good to know. Currently we have capture_headers disabled for the service in question. Does that copy work in an case-insensitive way?

I can suggest you to try the following workaround for now:

* Capture your override for user_agent as a normal label via the public API

* Add a [custom ingest pipeline](https://www.elastic.co/guide/en/apm/guide/current/ingest-pipelines.html#custom-ingest-pipelines) for moving the value from your custom label into `user_agent.original`

* Your custom ingest pipeline will have to redo the parsing and generation of the other `user_agent.*` fields, but you should be able to copy that from the standard APM ingest pipeline

We also would be willing to accept a PR for adding a Transaction.setHeader() method to the public-API, though es explained earlier we would prefer to not extend it, as we'll have to continue to support it.

This sounds like a few too many custom steps IMO, so I'd prefer to work on a PR. It should probably be Transaction.addHeader() instead of Transaction.setHeader()?

[jackshirazi]

Does that copy work in an case-insensitive way?

Yes, if you look at the extracted headers it just extracts the strings as they are

It should probably be Transaction.addHeader() instead of Transaction.setHeader()?

I think setting the header is right rather than appending. Since this would be for customized values, it should be entirely defined by the passed value