Enable custom istio metric tags with Telemetry API

[song-william]

Pull Request Summary

We can now tag metrics at the istio-level with API-routes. Istio-level metrics serve as a better source of truth for request counts.

We are using the alpha Telemetry APIs here: https://istio.io/latest/docs/tasks/observability/metrics/classify-metrics/
The actual routes were generated with an internal script from the OpenAPI service spec. The script will be open-sourced later.

Test Plan and Usage Guide

Applied the new yamls to test cluster. Pinged the service a few times and observed new tags in prometheus metrics
Command

kubectl exec model-engine-54f96d7f5c-65gmz -c istio-proxy -- curl -sS 'localhost:15000/stats/prometheus' > prom-mod-new.txt

New metric example, notice request_operation tag

istio_requests_total{response_code="200",reporter="destination",request_operation="get_/v1/llm/model-endpoints/_model_endpoint_name", ...} 2