[Serve] change the metric tag for the proxy metrics to `route_prefix` for clarity

[abrarsheikh]

We could consider changing the metric tag for the proxy metrics to route_prefix for clarity, but that introduces a migration for what seems to me like a very minor improvement.

Originally posted by @edoakes in #47999

the route tag on proxy metrics is causing confusion to some users. Let's migrate it to route_prefix incrementally

1. dual write values to both route_prefix and route for 2 versions.
2. update doc indicating the users should be using route_prefix and support for route will in dropped in 2 versions
3. update ray/serve dashboards that depend on route
4. after two version stop emitting route tag from proxy metrics

[lunce-lab]

Definitely agreed that this causes confusion.
route_prefix can either be / or /-/healthz based on current implementation of proxy metric. Grouping by this tag doesn't provide much helpful information.
In comparison, metrics instrumented by python/ray/serve/metrics.py for example ray_serve_deployment_request_counter_total contains real route path info, like /endpoint1, /endpoint2.
Can we add the real route tag in the metrics instead of deprecating it?

[abrarsheikh]

@lunce-lab that is how it was previously implemented, but we retracted from that because of #47999 (comment)