Add relabel config to plot durations for each gRPC method

[nakabonne]

What this PR does / why we need it:
For durations, stats reported by Envoy is enough to see.

Which issue(s) this PR fixes:

Fixes #

Does this PR introduce a user-facing change?:

NONE

[pipecd-bot]

Code coverage for golang is 32.26%. This pull request does not change code coverage.

[khanhtc1202]

/lgtm

[nghialv]

Could you add some metrics samples here? (e.g. envoy_cluster_grpc_*_upstream_rq_time_bucket)
That helps to review the use of regular expressions.

[nakabonne]

@nghialv You're right.

Some examples here:

# TYPE envoy_cluster_grpc_service_webservice_WebService_GetInsightApplicationCount_upstream_rq_time histogram
envoy_cluster_grpc_service_webservice_WebService_GetInsightApplicationCount_upstream_rq_time_bucket{envoy_grpc_bridge_method="api",envoy_grpc_bridge_service="pipe",envoy_cluster_name="server-web-api",le="0.5"} 0
envoy_cluster_grpc_service_webservice_WebService_GetInsightApplicationCount_upstream_rq_time_sum{envoy_grpc_bridge_method="api",envoy_grpc_bridge_service="pipe",envoy_cluster_name="server-web-api"} 2770
envoy_cluster_grpc_service_webservice_WebService_GetInsightApplicationCount_upstream_rq_time_count{envoy_grpc_bridge_method="api",envoy_grpc_bridge_service="pipe",envoy_cluster_name="server-web-api"} 6

# TYPE envoy_cluster_grpc_service_webservice_WebService_GetProject_upstream_rq_time histogram
envoy_cluster_grpc_service_webservice_WebService_GetProject_upstream_rq_time_bucket{envoy_grpc_bridge_method="api",envoy_grpc_bridge_service="pipe",envoy_cluster_name="server-web-api",le="0.5"} 0
envoy_cluster_grpc_service_webservice_WebService_GetProject_upstream_rq_time_sum{envoy_grpc_bridge_method="api",envoy_grpc_bridge_service="pipe",envoy_cluster_name="server-web-api"} 10.5
envoy_cluster_grpc_service_webservice_WebService_GetProject_upstream_rq_time_count{envoy_grpc_bridge_method="api",envoy_grpc_bridge_service="pipe",envoy_cluster_name="server-web-api"} 1

[nghialv]

Should we add envoy to the name? for example, pipecd_envoy_requests_

[nakabonne]

for sure, or pipecd_gateway_requests_ seems to be better

[nakabonne]

because this is a relabel_config for the pipecd-gateway job

[nghialv]

Right. That is better. 👍

[nakabonne]

Thank you for looking over. Applied them!

[nghialv]

Nice.
/lgtm

[pipecd-bot]

Code coverage for golang is 32.26%. This pull request does not change code coverage.

[khanhtc1202]

/approve

[pipecd-bot]

This pull request is APPROVED by khanhtc1202.

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