Add support for grpc health check

[averzicco]

Hi, This PR is to add support for gRPC health check to blackbox exporter.
I've read the discussion on issues #665 and #603, it seems that gRPC is out of scope for blackbox_exporter, if this is still the case just close the PR.

[roidelapluie]

Hello, it is in scope again, as discussed during our public dev summits. I will review this asap.

[averzicco]

great, I've checked the dev summit video and realized didn't implement the logic to specify the service name for the health check. I've just pushed the code which takes care of that

[brancz]

Really awesome that you're taking this on, been wanting this for a long time but haven't had a chance to implement it! Thank you so much!

[roidelapluie]

Can you please add a probe_grpc_duration_seconds with phase="check" ?

[roidelapluie]

I am puzzled by the fact that we can not know the difference between SERVICE_UNKNOWN, UNKNOWN, and NOT_SERVING. Would we need an additional metric for health check response status?

[averzicco]

I am puzzled by the fact that we can not know the difference between SERVICE_UNKNOWN, UNKNOWN, and NOT_SERVING. Would we need an additional metric for health check response status?

what do you prefer for the metric definition?
a gauge with status label

probe_grpc_response{status="UNKNOWN"}=0
probe_grpc_response{status="SERVING"}=1
probe_grpc_response{status="NOT_SERVING"}=0
probe_grpc_response{status="SERVICE_UNKNOWN"}=0

or a gauge with status as numeric value (same as value used in gRPC healtCheck proto)

probe_grpc_response=1

[roidelapluie]

Since the numbers are pretty well defined in the proto, I'd go for the probe_grpc_healthcheck_serving_status=1.

[averzicco]

Metric probe_grpc_healthcheck_serving_status added

[roidelapluie]

After checking with others, it seems it would be preferable to have

probe_grpc_healthcheck_response{serving_status="UNKNOWN"}=0
probe_grpc_healthcheck_response{serving_status="SERVING"}=1
probe_grpc_healthcheck_response{serving_status="NOT_SERVING"}=0
probe_grpc_healthcheck_response{serving_status="SERVICE_UNKNOWN"}=0

[averzicco]

Changed metric from probe_grpc_healthcheck_serving_status=1 to

probe_grpc_healthcheck_response{serving_status="UNKNOWN"}=0
probe_grpc_healthcheck_response{serving_status="SERVING"}=1
probe_grpc_healthcheck_response{serving_status="NOT_SERVING"}=0
probe_grpc_healthcheck_response{serving_status="SERVICE_UNKNOWN"}=0

[roidelapluie]

LGTM besides name change.

[roidelapluie]

Thanks!

[autumnw]

Appreciate to all the people who joined conversation and contribute code here! This is so awesome!
Kudos to @averzicco!