Add support for counter metric with external supplied value

[fab-10]

PR description

Another PR of the series to upgrade and refresh the metric system.
This time we fix the type of two metrics used in the MonitoredExecutors class, that are counters but were created as gauges, probably since there was no way to create counters with an external supplied value, so this kind of counter has been added, and the concept of metrics with external supplied values has been extended to cover both counters and gauges.

Other than for using the correct metric type, this fix is required, because with the following upgrade to Prometheus Java lib 1.x, only counters can have a _total suffix.

Fixed Issue(s)

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