mgr/prometheus: Add healthcheck metric for SLOW_OPS

mgr/prometheus: Add healthcheck metric for SLOW_OPS

SLOW_OPS is triggered by op tracker, and generates a health
alert but healthchecks do not create metrics for prometheus to
use as alert triggers. This change adds SLOW_OPS metric, and
provides a simple means to extend to other relevant health
checks in the future

If the extract of the value from the health check message fails
we log an error and remove the metric from the metric set. In
addition the metric description has changed to better reflect
the scenarios where SLOW_OPS can be triggered.

Signed-off-by: Paul Cuzner <pcuzner@redhat.com>