Changed zio metric type for active requests from Count to Gauge.

[Grryum]

A counter is not a good way to represent requests active metrics, as many platforms handle counters differently than a gauge. It seems like keeping track of the active requests should be represented as a gauge.

According to:

Prometheus docs: https://prometheus.io/docs/concepts/metric_types/#counter

Do not use a counter to expose a value that can decrease. For example, do not use a counter for the number of currently running processes; instead use a gauge.

Datadog docs: https://docs.datadoghq.com/metrics/types/?tab=count

The COUNT metric submission type represents the total number of event occurrences in one time interval. A COUNT can be used to track the total number of connections made to a database or the total number of requests to an endpoint.

[kciesielski]

Thanks a lot, @Grryum!