Per-node Micrometer duration histograms in MicrometerAgentListener

[ahmed-sekka]

MicrometerAgentListener currently emits counters (executions, retries, errors). For production observability we want per-node duration histograms so dashboards can show p50 / p95 / p99 latency per agent in a graph.

Acceptance criteria

• MicrometerAgentListener records a Timer per node, tagged with node, graph, outcome (success / failed / interrupted)
• Tests verify metrics are registered with the expected tags
• Section in docs/observability.md (new or existing) shows a Grafana panel example

Suggested metric naming

• agentflow.node.duration (timer, new)
• agentflow.node.executions (counter, already exists)