Monitor deprecated API usage via Prometheus metrics

[psrok1]

Your checklist for this pull request

• I've read the contributing guideline.
• I've tested my changes by building and running the project, and testing changed functionality (if applicable)
• I've added automated tests for my change (if applicable, optional)
• I've updated documentation to reflect my change (if applicable)

What is the current behaviour?

MWDB Core has few deprecated features that are waiting for removal. To be sure that we won't break anything, we need to monitor usage of these features.

What is the new behaviour?

Previously monitoring was done via logger.warning but it appeared to be pretty inefficient with current amount of deprecated things. In addition, not all deprecated features have been covered. This PR notifies about deprecated API usage via Prometheus metrics (introduced in #908) and debug-level logs.

Another important change is rename from api_requests to mwdb_api_requests, it's more convenient to have prefixed metric names.