Add documentation for activator_autoscaler_reachable metric

[prashanthjos]

This PR adds documentation for the new activator_autoscaler_reachable metric that was added in knative/serving.

Changes

Added documentation for the activator_autoscaler_reachable metric to the
Activator section in docs/versioned/serving/observability/metrics/serving-metrics.md.

Metric Details

• Name: activator_autoscaler_reachable
• Type: Int64Gauge
• Unit: {reachable}
• Values: 1 (reachable), 0 (not reachable)
• Description: Whether the autoscaler is reachable from the activator

Related PRs

knative/serving#16318

[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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• docs/versioned/serving/OWNERS

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[knative-prow]

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

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