Make it possible to view and debug the lifecycle of a secret populated by the kubernetes_secrets provider

[cmacknz]

The agent kubernetes_secrets provider has an internal cache that updates periodically and includes a TTL. https://www.elastic.co/guide/en/fleet/current/kubernetes_secrets-provider.html

The secrets provider includes limited logging about when individual secrets are updated or expired. The values populated by this provider should also be considered secrets in diagnostics and be redacted, making it challenging or impossible to tell if a secret updated when it was supposed to.

When the cache updates today for example, only a single log line indicating that some unindicated secret value was updated or deleted without specifying which is logged

elastic-agent/internal/pkg/composable/providers/kubernetessecrets/kubernetes_secrets.go

Lines 112 to 116 in 56901ed

	updatedCache := p.updateCache()
	if updatedCache {
	p.logger.Info("Secrets cache was updated, the agent will be notified.")
	comm.Signal()
	}

We recently had an internal case where a JWT token populated as a kubernetes secret in the agent policy did not rotate as expected, and the limited logging in agent made it impossible to tell if agent was involved in the root cause.

Add logging that would allow us to verify that agent is updating secrets at the expected times with the correct values in the policy, without leaking the actual secret values.

[elasticmachine]

Pinging @elastic/elastic-agent-control-plane (Team:Elastic-Agent-Control-Plane)

[adamkasztenny]

Maybe the hash of the secret could be logged? Elasticsearch does something similar for the JWKS here (although admittedly that's not necessarily secret data, the JWKS could be publicly accessible). This could be an optional setting that is disabled by default.