Extend metrics so that 404s are properly reported

[adamw]

Plus, document that the EndpointMetric callbacks are exclusive - exactly one will be called for each metric, after a request is handled (this assumes a new callback is added, when no endpoints match)

[adamw]

In fact, this might not be needed - .defaultHandlers(msg => ..., notFoundWhenRejected = true) should be enough.

In that case, the tracing interceptor might need to be changed, to remove the noEndpointAttributes