frontend: lint Prometheus metrics

[simonpasquier]

What this PR does

This change renames the frontend_count metric to frontend_requests_total to comply with the Prometheus guidelines.

It also bounds the cardinality of the route label by using the matched pattern instead of the actual path.

Jira:
Link to demo recording:

Before this PR

# TYPE frontend_count counter
frontend_count{api_version="",code="200",route="/healthz",state="Unknown",verb="GET"} 2
frontend_count{api_version="",code="404",route="/notfound",state="Unknown",verb="GET"} 1
frontend_count{api_version="",code="404",route="/notfound2",state="Unknown",verb="GET"} 1
frontend_count{api_version="",code="404",route="/notfound3",state="Unknown",verb="GET"} 1
frontend_count{api_version="2.0",code="201",route="/subscriptions/1d3378d3-5a3f-4712-85a1-2485495dfc4b",state="Registered",verb="PUT"} 1
frontend_count{api_version="2024-06-10-preview",code="200",route="/subscriptions/1d3378d3-5a3f-4712-85a1-2485495dfc4b/resourcegroups/simon-net-rg/providers/microsoft.redhatopenshift/hcpopenshiftclusters",state="Registered",verb="GET"} 2
frontend_count{api_version="2024-06-10-preview",code="404",route="/subscriptions/1d3378d3-5a3f-4712-85a1-2485495dfc4b/resourcegroups/simon-net-rg/providers/microsoft.redhatopenshift/hcpopenshiftclusters/simon",state="Registered",verb="GET"} 2
# HELP frontend_duration 
# TYPE frontend_duration gauge
frontend_duration{api_version="",code="200",route="/healthz",verb="GET"} 0
frontend_duration{api_version="",code="404",route="/notfound",verb="GET"} 0
frontend_duration{api_version="",code="404",route="/notfound2",verb="GET"} 0
frontend_duration{api_version="",code="404",route="/notfound3",verb="GET"} 0
frontend_duration{api_version="2.0",code="201",route="/subscriptions/1d3378d3-5a3f-4712-85a1-2485495dfc4b",verb="PUT"} 1
frontend_duration{api_version="2024-06-10-preview",code="200",route="/subscriptions/1d3378d3-5a3f-4712-85a1-2485495dfc4b/resourcegroups/simon-net-rg/providers/microsoft.redhatopenshift/hcpopenshiftclusters",verb="GET"} 431
frontend_duration{api_version="2024-06-10-preview",code="404",route="/subscriptions/1d3378d3-5a3f-4712-85a1-2485495dfc4b/resourcegroups/simon-net-rg/providers/microsoft.redhatopenshift/hcpopenshiftclusters/simon",verb="GET"} 0
# HELP frontend_health 
# TYPE frontend_health gauge
frontend_health{endpoint="/healthz"} 1

After this PR (note how all the 404 paths are folded into the / route):

# HELP frontend_duration 
# TYPE frontend_duration gauge
frontend_duration{api_version="",code="200",route="/healthz",verb="GET"} 5.8412e-05
frontend_duration{api_version="",code="404",route="/",verb="GET"} 2.7196e-05
frontend_duration{api_version="2.0",code="201",route="/subscriptions/{subscriptionid}",verb="PUT"} 0.000267956
frontend_duration{api_version="2024-06-10-preview",code="200",route="/subscriptions/{subscriptionid}/resourcegroups/{resourcegroupname}/providers/microsoft.redhatopenshift/hcpopenshiftclusters",verb="GET"} 0.829475008
frontend_duration{api_version="2024-06-10-preview",code="400",route="/subscriptions/{subscriptionid}/resourcegroups/{resourcegroupname}/providers/microsoft.redhatopenshift/hcpopenshiftclusters/{resourcename}",verb="GET"} 0.000129962
frontend_duration{api_version="2024-06-10-preview",code="404",route="/subscriptions/{subscriptionid}/resourcegroups/{resourcegroupname}/providers/microsoft.redhatopenshift/hcpopenshiftclusters/{resourcename}",verb="GET"} 0.000103378
# HELP frontend_health 
# TYPE frontend_health gauge
frontend_health{endpoint="/healthz"} 1
# HELP frontend_requests_total 
# TYPE frontend_requests_total counter
frontend_requests_total{api_version="",code="200",route="/healthz",state="Unknown",verb="GET"} 1
frontend_requests_total{api_version="",code="404",route="/",state="Unknown",verb="GET"} 1
frontend_requests_total{api_version="2.0",code="201",route="/subscriptions/{subscriptionid}",state="Registered",verb="PUT"} 1
frontend_requests_total{api_version="2024-06-10-preview",code="200",route="/subscriptions/{subscriptionid}/resourcegroups/{resourcegroupname}/providers/microsoft.redhatopenshift/hcpopenshiftclusters",state="Registered",verb="GET"} 1
frontend_requests_total{api_version="2024-06-10-preview",code="400",route="/subscriptions/{subscriptionid}/resourcegroups/{resourcegroupname}/providers/microsoft.redhatopenshift/hcpopenshiftclusters/{resourcename}",state="Unknown",verb="GET"} 1
frontend_requests_total{api_version="2024-06-10-preview",code="404",route="/subscriptions/{subscriptionid}/resourcegroups/{resourcegroupname}/providers/microsoft.redhatopenshift/hcpopenshiftclusters/{resourcename}",state="Registered",verb="GET"} 2
...

Special notes for your reviewer

[github-actions]

Please rebase pull request.

[simonpasquier]

@SudoBrendan @mbarnes could you please review?

[github-actions]

Please rebase pull request.

[mbarnes]

Sorry for the delay on this, I haven't been watching incoming PRs closely.
This is on my radar now. If you can resolve the current conflicts I'll give it a review ASAP.

[mbarnes]

This is a fantastic use of http.Request.Pattern, which I didn't even know existed! Changes look great, thank you.