fix(kuma-cp) metrics hijacker incorrect fallback during merge

[lobkovilya]

Summary

Metrics Hijacker could merge only metrics with the same label values. This's an incorrect way to handle data. Different label vales is a valid use case:

# TYPE envoy_cluster_external_upstream_rq counter
envoy_cluster_external_upstream_rq{envoy_response_code="200",envoy_cluster_name="backend_kuma-demo_svc_3001-_0_"} 14
envoy_cluster_external_upstream_rq{envoy_response_code="404",envoy_cluster_name="backend_kuma-demo_svc_3001-_0_"} 2
envoy_cluster_external_upstream_rq{envoy_response_code="200",envoy_cluster_name="backend_kuma-demo_svc_3001-_1_"} 24
envoy_cluster_external_upstream_rq{envoy_response_code="404",envoy_cluster_name="backend_kuma-demo_svc_3001-_1_"} 12
envoy_cluster_external_upstream_rq{envoy_response_code="200",envoy_cluster_name="kuma_envoy_admin"} 255

should become:

# TYPE envoy_cluster_external_upstream_rq counter
envoy_cluster_external_upstream_rq{envoy_response_code="200",envoy_cluster_name="backend_kuma-demo_svc_3001"} 38
envoy_cluster_external_upstream_rq{envoy_response_code="404",envoy_cluster_name="backend_kuma-demo_svc_3001"} 14
envoy_cluster_external_upstream_rq{envoy_response_code="200",envoy_cluster_name="kuma_envoy_admin"} 255

Current PR changes the way Hijacker merges the metrics. Also, added comments on how it works.

Full changelog

• add more test cases
• replace 'merge' algorithm
• add comments

Issues resolved

N/A

Documentation

• Link to the website documentation PR

Testing

• Unit tests
• E2E tests
• Manual testing on Universal
• Manual testing on Kubernetes

Backwards compatibility

• Add backport-to-stable label if the code is backwards compatible. Otherwise, list breaking changes.