Adapt rules to use new gRPC metrics

[brancz]

This adapts the provided example alerting rules for Prometheus to use the new gRPC metrics coming from grpc-go-prometheus and fixes the incorrect link introduced in #8848 (comment).

@xiang90 @gyuho

[codecov-io]

Codecov Report

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@@            Coverage Diff            @@
##             master    #8879   +/-   ##
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  Branches          ?        0           
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  Hits              ?    22598           
  Misses            ?     5538           
  Partials          ?     1583

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[gyuho]

lgtm. thanks! /cc @xiang90

[xiang90]

lgtm