benchmarking: add memory working set to server telemetry

[Nishanth Kotla (Nishanth29)]

Depends on #2221. The first 3 commits are #2221's; this PR's change is only the last commit, 36d26077 ("benchmarking: add memory working set to server telemetry"). Will rebase once #2221 merges.

Part of #1590

Adds working_set to server telemetry:

• node, ateom, atelet: from cAdvisor (container_memory_working_set_bytes).
• actor, per_actor: reuse the atelet's existing actor stats metrics,
  ate_actor_stats_memory_working_set_bytes and ate_actor_stats_sampled_actors.
  actor is the actors' working set per atelet; per_actor divides it by
  the number of actors that atelet measured.

Both sources are already collected (cAdvisor and the atelet), so nothing changes in Substrate or the monitoring setup; this only adds queries.

Live check (gVisor, 4 users, 3m, through the telemetry-meter):

• actor 1.13 GiB vs ateom 1.22 GiB at the same time

• per_actor × count = actor (0.5628 × 2 = 1.1257)

• 74 measurements, none null

• Tests pass

• Appropriate changes to documentation are included in the PR

[Nishanth Kotla (Nishanth29)]

cc Haowei Cai (Roy) (@roycaihw), Max Smythe (@maxsmythe) - ptal

[Nishanth Kotla (Nishanth29)]

Da Huang (@git286) Tim Bai (@baizhenyu), adding you as reviewers. For actors this only reads your existing ate.actor.stats.memory.working_set and ate.actor.stats.sampled_actors (no new metrics); node/ateom/atelet come from cAdvisor.