Metrics for MSC3861 token introspection

[hughns]

Off the back of #16087 and given the dependency between Synapse and a MSC3861 OpenID Provider on an ongoing basis, it probably makes sense to expose metrics for:

• number of introspection requests
• response time
• cache hit/miss rates

[sandhose]

I would implement this as an histogram around the synapse.api.msc3861_delegated.MSC3861Delegated#_introspect_token method.
It should have labels to distinguish at least those states:

• cache hit
• cache miss, but non-2xx status code
• cache miss, and introspection call succeeded

[erikjohnston]

Have added a response time histogram in #16131, which is labeled by response code (matching other such histograms).

We should already have cache hit/miss counter metrics by default, so I don't think we need any thing else there