Add Bitbucket Importer metrics

What does this MR do?

This MR:

• Introduces new module Gitlab::Import::Metrics to be prepended to Importers in order to define metrics to be emitted at the top of the class
• Adds metrics to Bitbucket Importer in order to start collecting metrics around this importer
• Next MRs are going to implement similar metrics to the rest of the available Importers (Gitea, Fogbugz, etc)

I went with this approach mainly because I did not wish to add Gitlab::Metrics.counter(...).increment & Gitlab::Metrics.histogram(...).observe inline with the rest of business logic and to have all metrics defined in one place.

Once the metrics are emitted we can then build Grafana dashboards.

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team