add RequestMetricsMiddleware which tracks key metrics related to each request the LLS server will recieve:
1. llama_stack_requests_total: tracks the total amount of requests the server has processed
2. llama_stack_request_duration_seconds: tracks the duration of each request
3. llama_stack_concurrent_requests: tracks concurrently processed requests by the server
The usage of a middleware allows this to be done on the server level without having to add custom handling to each router like the inference router has today for its API specific metrics.
Also, add some unit tests for this functionality
resolves#2597
Signed-off-by: Charlie Doern <cdoern@redhat.com>
