Remove route and method tags from rate limiting

[JamesNK]

This PR removes route and method tags from rate-limiting metrics.

Three reasons:

• Route information isn't specific to rate limiting. It doesn't fit (unless we decide that all asp.net core metrics should have this information).
• Rate limiting has a policy name to identify rate-limiting metrics. For example, you can see a count of queued requests against a policy name.
• We can add route tags back in the future if there is feedback that it's wanted.

[mitchdenny]

Makes sense to me.