Add metric and logging for activator-autoscaler connectivity

[prashanthjos]

Description

This PR adds observability for the websocket connection between the activator and autoscaler components. When the autoscaler is not reachable, operators currently have no easy way to identify this issue, which can lead to autoscaling failures.

Changes

New Metric

• Name: kn.activator.autoscaler.reachable
• Type: Int64Gauge
• Values: 1 (reachable), 0 (not reachable)
• Description: Whether the autoscaler is reachable from the activator

New Logging

• ERROR level log when autoscaler is not reachable:
  • "Autoscaler is not reachable from activator. Stats were not sent." (on send failure)
  • "Autoscaler is not reachable from activator." (on connection check failure)

How It Works

The metric is recorded in two scenarios:

1. Periodic check (every 5s):

   • Uses [conn.Status()] to check if connection is established
   • Catches: Connection never established, DNS failures, autoscaler not running at startup

2. On stat send:

   • Detects [SendRaw()] failures during actual stat message transmission
   • Catches: Network timeouts, connection drops, autoscaler becoming unreachable

Testing

• Unit tests pass (go test ./pkg/activator/... -v)
• New test TestAutoscalerConnectionStatusMonitor added

[knative-prow]

Welcome @prashanthjos! It looks like this is your first PR to knative/serving 🎉

[knative-prow]

Hi @prashanthjos. Thanks for your PR.

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[knative-prow]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: prashanthjos
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Details

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[thiagomedina]

/ok-to-test

[codecov]

Codecov Report

❌ Patch coverage is 83.33333% with 7 lines in your changes missing coverage. Please review.
✅ Project coverage is 80.08%. Comparing base (a8803aa) to head (1e54a49).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
pkg/activator/stat_reporter.go	80.00%	4 Missing and 1 partial ⚠️
pkg/activator/metrics.go	88.23%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #16318      +/-   ##
==========================================
- Coverage   80.09%   80.08%   -0.02%     
==========================================
  Files         215      216       +1     
  Lines       13391    13429      +38     
==========================================
+ Hits        10725    10754      +29     
- Misses       2304     2312       +8     
- Partials      362      363       +1     

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[prashanthjos]

/retest

[prashanthjos]

Related docs PR:

knative/docs#6548