Do not set explicit unit on Prometheus 1.x Metadata

[jonatan-ivanov]

We should not set Unit in the Metadata since if it is set, it must be a suffix of the MetricFamily name separated by an underscore (see OpenMetrics spec).

This can be problematic since Micrometer adds the unit to the name but not necessarily as a suffix. For example, Micrometer does this test_seconds_max but if Unit is present the name should be test_max_seconds. The difference is not having the invalid unit definition on the output (# UNIT test_seconds_max seconds) which would cause scraping errors on the server side (the Prometheus 0.x registry also does this).