Add true RSS reporting to dashboards

[chrisbliss18]

Summary

This closes out the dashboard memory reporting work by adding true process RSS alongside the existing Go runtime memory signal.

The host and fleet dashboards now expose both values so operators can compare Jetmon with host-level tools like ps, top, and systemd while still keeping Go Sys memory visible for runtime guardrails and pprof investigations.

What changed

• Added a shared internal/processmetrics helper for local process memory sampling.
• Added rss_mem_mb to jetmon_process_health via migration 26.
• Published RSS and Go Sys memory from both jetmon2 and jetmon-deliverer process-health snapshots.
• Updated /api/state, /api/host, /api/fleet, the host dashboard, and the fleet dashboard to expose RSS beside Go Sys memory.
• Updated StatsD memory gauges so process.rss_mb reports resident memory when procfs is available, with additive process.go_sys_mem_mb for the runtime allocator view.
• Updated operations, migration, architecture, data-model, project, changelog, and roadmap docs.

Example dashboard values

Host dashboard throughput section now shows memory as two separate cards:

RSS Memory      37MB
Go Sys Memory   24MB

Fleet process rows now show both values in the memory column:

host-a:monitor     green   running   2s   0-499   active=12 queue=4 retry=0   rss=37MB go=24MB
host-b:deliverer   green   running   3s   -       active=0 queue=0 retry=0    rss=19MB go=14MB

The JSON payload exposes both fields for machine parsing:

{
  "go_sys_mem_mb": 24,
  "rss_mem_mb": 37
}

Review notes

I reviewed the branch from sysadmin, security, and code-quality perspectives after the initial implementation.

• Sysadmin: RSS is now the first memory value shown, with Go Sys retained as the runtime and guardrail signal.
• Security: RSS collection reads only /proc/self/statm; there is no user-controlled path in production code and no additional remote surface beyond the existing internal dashboards and APIs.
• Engineering: memory sampling now flows through one focused CurrentMemory() helper, and tests assert the exact JSON and StatsD fields operators and automation will consume.

Testing

• go test ./...
• go vet ./...
• make all
• Targeted dry-run style tests for procfs parsing, current memory sampling, /api/state JSON, fleet rollup preservation, and StatsD metric names.