Remove 1000 characters limit for http client request body capturing

[JonasKunz]

What does this PR do?

Removes the 1000 character limit for capturing http client bodies via the capture_http_client_request_body_size.
This required the sending to switch from using a label to a new IntakeV2 field added in apm-server 8.18.

For backwards compatibility, a new capture_http_client_request_body_as_label option was added.

Checklist

• This is an enhancement of existing features, or a new feature in existing plugins
  • I have updated CHANGELOG.next-release.md
  • I have added tests that prove my fix is effective or that my feature works
  • Added an API method or config option? Document in which version this will be introduced
  • I have made corresponding changes to the documentation

[JonasKunz]

Note that this needs either #4039 to be fixed before the next release or we'll have to update the docs manually on release.

[JonasKunz]

See #4060

[JonasKunz]

Manually tested on an 8.18 cluster in addition to the unit tests.

With the capture_http_client_request_body_as_label flag on default (disabled):

With the flag enabled:

[SylvainJuge]

[nit] I think we tend to rely more on traditional for loop to avoid allocating an iterator.