tweak activator stats reporting metrics

[dprotaso]

• Change metrics name to have a common prefix 'kn.activator.stats.conn.*'
• Encapsulate OTel details into metrics struct

[dprotaso]

/assign @prashanthjos @linkvt

Thinking about the metric names a bit more - I think they should be a bit more explicit they they relate to stats reporting connection status

[knative-prow]

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/assign @prashanthjos @linkvt

Thinking about the metric names a bit more - I think they should be a bit more explicit they they relate to stats reporting connection status

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[dprotaso]

Looks like there's a flake

panic: Log in goroutine after TestReportStatsSendFailure has completed: 2026-01-15T18:28:23.624Z	ERROR	activator/stat_reporter.go:68	Autoscaler is not reachable from activator. Stats were not sent.	{"error": "connection refused", "stat_message_count": 1}
	

goroutine 9 [running]:
testing.(*common).log(0xc000170c40, {0xc0002680c0, 0xb6})
	/opt/hostedtoolcache/go/1.25.5/x64/src/testing/testing.go:1030 +0x1df
testing.(*common).Logf(0xc000170c40, {0x1639105, 0x2}, {0xc00004f900, 0x1, 0x1})
	/opt/hostedtoolcache/go/1.25.5/x64/src/testing/testing.go:1191 +0x8f
go.uber.org/zap/zaptest.TestingWriter.Write({{0x1826530?, 0xc000170c40?}, 0xf0?}, {0xc000199400, 0xb7, 0x400})
	/home/runner/work/serving/serving/vendor/go.uber.org/zap/zaptest/logger.go:146 +0x11a
go.uber.org/zap/zapcore.(*ioCore).Write(0xc0002338c0, {0x2, {0xc2526a51e53a3648, 0x117105d, 0x1faec80}, {0x0, 0x0}, {0x1682d1c, 0x40}, {0x1, ...}, ...}, ...)
	/home/runner/work/serving/serving/vendor/go.uber.org/zap/zapcore/core.go:99 +0x18e
go.uber.org/zap/zapcore.(*CheckedEntry).Write(0xc000211380, {0xc000167a80, 0x2, 0x2})
	/home/runner/work/serving/serving/vendor/go.uber.org/zap/zapcore/entry.go:258 +0x1ed
go.uber.org/zap.(*SugaredLogger).log(0xc00006e648, 0x2, {0x1682d1c, 0x40}, {0x0, 0x0, 0x0}, {0xc000141f18, 0x2, 0x2})
	/home/runner/work/serving/serving/vendor/go.uber.org/zap/sugar.go:355 +0x12d
go.uber.org/zap.(*SugaredLogger).Errorw(...)
	/home/runner/work/serving/serving/vendor/go.uber.org/zap/sugar.go:269
knative.dev/serving/pkg/activator.ReportStats.func1({0xc000036660, 0x1, 0x1?})
	/home/runner/work/serving/serving/pkg/activator/stat_reporter.go:68 +0x7c9
created by knative.dev/serving/pkg/activator.ReportStats in goroutine 8
	/home/runner/work/serving/serving/pkg/activator/stat_reporter.go:59 +0xa5
FAIL	knative.dev/serving/pkg/activator	0.035s

[nader-ziada]

/lgtm

[codecov]

Codecov Report

❌ Patch coverage is 30.76923% with 9 lines in your changes missing coverage. Please review.
✅ Project coverage is 80.08%. Comparing base (ca98904) to head (d4d5f11).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
pkg/activator/metrics.go	30.00%	7 Missing ⚠️
pkg/activator/stat_reporter.go	33.33%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #16341      +/-   ##
==========================================
+ Coverage   80.06%   80.08%   +0.01%     
==========================================
  Files         216      216              
  Lines       13429    13436       +7     
==========================================
+ Hits        10752    10760       +8     
+ Misses       2317     2316       -1     
  Partials      360      360              

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[prashanthjos]

/retest

[knative-prow]

@prashanthjos: changing LGTM is restricted to collaborators

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[knative-prow]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: dprotaso, prashanthjos

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