NewHandlerFactory is unfortunately registering its metrics in the
global registry of Prometheus client library. This also leads to
its tests leaking state between different test cases as they all
access the same registry. This has been worked around by applying
a namespace on the metric names of each test.

Isolate the tests correctly by overriding the global registry with
a test specific one, and recovering the original registry at the
end of the test. This way each of the tests can access have their
own isolated registry no longer need namespaces to avoid conflicts.

As we're no longer implicitly testing the namespace option through
using it for test isolation, we add an explicit test to assert the
functionality works.

NewHandlerFactory registers its metrics directly into the global
registry for the Prometheus client library. This forces global state
on applications that use the shared functionality from LabKit. Global
state is discouraged as it makes testing difficult, the application
code inflexible and more complex as it couples unrelated aspects of an
application together.

This commit adds an option to provide a custom registerer where the
factory will register metrics. This way the application has an option
to avoid the global state of the default registry. It would be better
to leave the metric registration up to the application entirely but
I've opted to stick with the existing pattern here and provide an
option that fits in the existing API.

Add an option for providing a custom prometheus.Registerer

See merge request !243



Merged-by: Andrew Newdigate <andrew@gitlab.com>
Approved-by: Elliot Forbes <eforbes@gitlab.com>
Approved-by: Andrew Newdigate <andrew@gitlab.com>
Reviewed-by: Sami Hiltunen <shiltunen@gitlab.com>
Co-authored-by: Sami Hiltunen <shiltunen@gitlab.com>

Adds WithRegisterer option to `http_round_tripper.NewFactory` to allow
applications to avoid global registry state, matching the pattern
introduced in c448e786 for the handler factory.

This follows the style of the original commit that made similar changes to the handler,
keeping it concise while explaining what was added and why.

feat: http-roundtripper metrics: add option for providing a custom prometheus.Registerer

See merge request !248



Merged-by: João Pereira <jpereira@gitlab.com>
Approved-by: Elliot Forbes <eforbes@gitlab.com>
Approved-by: João Pereira <jpereira@gitlab.com>
Reviewed-by: GitLab Duo <gitlab-duo@gitlab.com>
Co-authored-by: Andrew Newdigate <andrew@gitlab.com>