Enable serving supervisor metrics

[brandond]

Proposed Changes

• Refactor agent supervisor listener startup and authn/authz to use upstream auth delegators to perform for SubjectAccessReview for access to metrics.
• Convert spegel and pprof handlers over to new structure.
• Promote --enable-pprof to agent flag to allow profiling agents. Access to the pprof endpoint now requires client cert auth, similar to the spegel registry api endpoint.
• Add --supervisor-metrics server flag that configures all cluster members (both servers and agents) to serve metrics at 0.0.0.0:6443/metrics.
• The pprof and metrics endpoints use same listener as spegel. Use of any one of the three features will cause the listener to be enabled.

This is required to expose supervisor metrics on rke2; on k3s the metrics will be the same as those currently available from kubelet and apiserver metrics endpoints

• https://github.com/rancher/rke2/issues/5786
• https://github.com/k3s-io/k3s/issues/10094

Types of Changes

enhancement

Verification

metrics:

1. Start servers with --supervisor-metrics
2. curl -vks --cert /var/lib/rancher/k3s/server/tls/client-admin.crt --key /var/lib/rancher/k3s/server/tls/client-admin.key https://node:6443/metrics against agents and servers

pprof:

1. Start node (agent or server) with --enable-pprof
2. curl -vks --cert /var/lib/rancher/k3s/server/tls/client-admin.crt --key /var/lib/rancher/k3s/server/tls/client-admin.key https://node:6443/debug/pprof/ against node with pprof enabled

Testing

Linked Issues

• https://github.com/k3s-io/k3s/issues/9951

User-Facing Change

`--enable-pprof` can now be set on agents to enable the debug/pprof endpoints. When set, agents will listen on the supervisor port.
`--supervisor-metrics` can now be set on servers to enable serving internal metrics on the supervisor endpoint; when set agents will listen on the supervisor port.

Further Comments

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 24.09091% with 167 lines in your changes are missing coverage. Please review.

Project coverage is 41.70%. Comparing base (dba30ab) to head (bcdd0be).

Files	Patch %	Lines
pkg/agent/https/https.go	0.00%	52 Missing :warning:
pkg/util/net.go	14.81%	45 Missing and 1 partial :warning:
pkg/profile/profile.go	0.00%	13 Missing :warning:
pkg/metrics/metrics.go	0.00%	9 Missing :warning:
pkg/agent/run.go	0.00%	4 Missing and 2 partials :warning:
pkg/cli/agent/agent.go	40.00%	3 Missing and 3 partials :warning:
pkg/cli/server/server.go	66.66%	3 Missing and 3 partials :warning:
pkg/spegel/spegel.go	0.00%	6 Missing :warning:
pkg/agent/config/config.go	58.33%	2 Missing and 3 partials :warning:
pkg/server/router.go	50.00%	5 Missing :warning:
... and 7 more

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #10019 +/- ## ========================================== - Coverage 46.55% 41.70% -4.85% ========================================== Files 173 177 +4 Lines 14645 14740 +95 ========================================== - Hits 6818 6148 -670 - Misses 6540 7425 +885 + Partials 1287 1167 -120 ``` | [Flag](https://app.codecov.io/gh/k3s-io/k3s/pull/10019/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=k3s-io) | Coverage Δ | | |---|---|---| | [e2etests](https://app.codecov.io/gh/k3s-io/k3s/pull/10019/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=k3s-io) | `36.38% <24.09%> (-10.00%)` | :arrow_down: | | [inttests](https://app.codecov.io/gh/k3s-io/k3s/pull/10019/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=k3s-io) | `37.05% <20.90%> (?)` | | | [unittests](https://app.codecov.io/gh/k3s-io/k3s/pull/10019/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=k3s-io) | `11.31% <0.90%> (-0.08%)` | :arrow_down: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=k3s-io#carryforward-flags-in-the-pull-request-comment) to find out more.

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