Migrate away from gauges where possible

[tsegismont]

Describe the feature

See #259

As suggested by Micrometer team, there are many gauges that can be replaced with long task timers.

Let's also track the outcome of micrometer-metrics/micrometer#6931 because the Micrometer team may come up with a new meter implementation.

Finally, when we get rid of the warning, we must remove the corresponding note in the docs