Fix GC::Profiler metrics fetching

What does this MR do?

Previously, both InfluxSampler and RubySampler were relying on the GC::Profiler.total_time data which is the sum over the list of captured GC events.

Also, both samplers asynchronously called GC::Profiler.clear which led to incorrect metric data because each sampler had the wrong assumption it was the only object who called GC::Profiler.clear and thus could rely on the gathered results between calls.

We should ensure that GC::Profiler.clear is called only in one place making it possible to rely on accumulated data between profiler wipes.

The approach suggested by Kamil: https://gitlab.com/gitlab-org/gitlab-ce/issues/63571#note_197621957

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry for user-facing changes, or community contribution. Check the link for other scenarios.
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Related to #63571 (closed)