[Metricbeat] Ability to specify prefix in HTTP module

[ymao1]

Describe the enhancement:

In the metricbeat HTTP module, we can specify a namespace, but this will direct the collected data to be namespaced under the http prefix (http.${namespace}. It would be useful to be able to change this prefix so it doesn't have to be written under http.

[botelastic]

This issue doesn't have a Team:<team> label.

[botelastic]

Hi!
We just realized that we haven't looked into this issue in a while. We're sorry!

We're labeling this issue as Stale to make it hit our filters and make sure we get back to it as soon as possible. In the meantime, it'd be extremely helpful if you could take a look at it as well and confirm its relevance. A simple comment with a nice emoji will be enough :+1.
Thank you for your contribution!