Remove import-specific metris from Gitlab::Import::Metrics

What does this MR do and why?

While working on gitlab-com/runbooks!4082 (merged) I noticed the opportunity to normalize some dashboard metrics for importers. Given that currently we're under-utilizing the dashboards, I think it better to normalize metrics now before we increate the usage of this data.

The goal of this MR is:

• remove project tag from github importer histogram, because it doesn't bring much value since this data is used as time vectors on prometheus/grafana.
• Use the same metrics for all importers that uses this class (Bitbucket, Github), this will help to normalize the dashboards and reduce the cognitive load from the developers, since all dashboards can have the same format.

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