Remove unused weak references

[fabianfett]

Remove unused weak prometheusClient references.

Checklist

• [x] The provided tests still run.
• [x] I've created new tests where needed.
• [x] I've updated the documentation if necessary.

Motivation and Context

Weak references are a performance bottleneck. Since the PrometheusHandled protocol is internal and not used at all, we can remove it and with this remove the weak references to the PrometheusClient. Less, but faster code.

Description

• Remove PrometheusHandled protocol
• Remove weak prometheusClient references in the metrics.

[ktoso]

Oh very nice, LGTM