fix(prometheus_sink): log errors and fallback to default metric mapper

[harpunius]

I recently migrated from statsd-exporter to the prometheus sink, and couldn't make the mapping work, until I found that the path didn't match up. This error is not logged currently, which is the main motivation for this PR.

I then thought that it would be nicer to fallback to the default mapper instead of having no mapping at all. Happy to rollback the default handling and simply add the error logs instead.

TestPrometheusSinkMetricMapperDefaultHandling is simply a test that would fail without these changes.