Fix monitoring metrics for individual collectors

[ananya-mallik-ps]

Fix monitoring metrics

Problem

Currently, each time a metrics endpoint is hit (with collect parameter e.g., /metrics?collect=pubsub.googleapis.com/topic), a new collector is created. This causes the monitoring metrics (stackdriver_monitoring_api_calls_total, stackdriver_monitoring_scrapes_total etc.) to reset on each scrape.

Solution

This PR:

• Add collector caching in the handler using a map
• Key the cache using project ID + filters combination
• Reuse collectors to maintain metric state across scrapes

Testing

• [x] Tested build locally, with one of our GCP project for different collector(s)
• [x] Deployed in one of our cluster and tested with multiple collector (~ 110+ different metrics-type-prefixes)
• [x] Verified metrics accumulate properly across scrapes for each collector
• [x] Memory usage remains stable over time and is similar to previous version

[ananya-mallik-ps]

@SuperQ @kgeckhart