Add Support for Querying Metrics from Google Managed Prometheus and Additional PromQL Filters now Configurable

[Bslabe123]

Addresses: #76

Adds metrics.prometheus.google_managed; when set to true, the Prometheus URL is automatically set to the Google Cloud Managed Service for Prometheus endpoint for the currently configured project. Additional PromQL filters are also now configurable via: metrics.prometheus.filters:

metrics:
  type: prometheus
  prometheus:
    google_managed: True
    scrape_interval: 15
    filters:
      - 'namespace="test-ns"'

Newly added logs:

2025-06-17 16:00:15,994 - inference_perf.client.metricsclient.prometheus_client.base - INFO - Prometheus metrics client configured, querying metrics from 'https://monitoring.googleapis.com/v1/projects/xxxxx/location/global/prometheus/api/v1/query'

...

2025-06-17 16:04:23,924 - inference_perf.client.metricsclient.prometheus_client.google_managed_prometheus_client - INFO - Making PromQL query: 'avg_over_time(vllm:num_requests_waiting{model_name='meta-llama/Meta-Llama-3-8B',namespace="test-ns"}[238s])'
2025-06-17 16:04:24,208 - inference_perf.client.metricsclient.prometheus_client.google_managed_prometheus_client - INFO - Making PromQL query: 'sum(rate(vllm:time_to_first_token_seconds_sum{model_name='meta-llama/Meta-Llama-3-8B',namespace="test-ns"}[238s])) / (sum(rate(vllm:time_to_first_token_seconds_count{model_name='meta-llama/Meta-Llama-3-8B',namespace="test-ns"}[238s])) > 0)'
2025-06-17 16:04:24,490 - inference_perf.client.metricsclient.prometheus_client.google_managed_prometheus_client - INFO - Making PromQL query: 'histogram_quantile(0.5, sum(rate(vllm:time_to_first_token_seconds_bucket{model_name='meta-llama/Meta-Llama-3-8B',namespace="test-ns"}[238s])) by (le))'
2025-06-17 16:04:24,757 - inference_perf.client.metricsclient.prometheus_client.google_managed_prometheus_client - INFO - Making PromQL query: 'histogram_quantile(0.9, sum(rate(vllm:time_to_first_token_seconds_bucket{model_name='meta-llama/Meta-Llama-3-8B',namespace="test-ns"}[238s])) by (le))'

[k8s-ci-robot]

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