Adding back PrometheusHandler to avoid breaking change

[rwkarg]

This adds a PrometheusHandler implementation to mimic the prior prometheus-net based implementation.

Note that the old implementation incorrectly created some labels/tags with a name of "method" but the value as the Verb of the request. This incorrect behavior is maintained and noted.

The end result is that the same named metrics will be produced. The prometheus-net library reads values from System.Diagnostics.Metrics already so these will be picked up by anyone that is already using this handler.

Ultimately, users should migrate to using the built in HttpClient metrics that covers these custom metrics, uses standard semantic conventions for metric names, and has the correct labels/tags added to metrics.

Addresses comment #1525 (comment)

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[rwkarg]

@brendandburns When you get a chance can you see if this addresses the concerns of #1525? ?

[brendandburns]

/lgtm
/approve

Thanks!

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[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: brendandburns, rwkarg

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