QFw: remeasure the instrumentation cost with the transport spans and logs

[DougSO]

Docs only: the "Instrumentation Cost" section of docs/design/benchmarking.md.

Why

The budget was measured on 2026-10-08 with three telemetry states, before the transport spans (#120), the logs tier (#123 to #136) and Loki (QFw-SLURM-Cluster #48) existed. It called full tracing "a benchmark or a demonstration", which the cluster overlay no longer runs: the overlay ships with the transport spans on and the logs tier at debug. The logs tier is also the first signal whose cost follows what a process logs rather than what a job does, and nobody had measured it.

What was measured

The extended harness (QFw-SLURM-Cluster #52) on the reference cluster, against the image of QFw main b82d19f: the same 300-job fake-IQM stream in seven states, each adding one thing to the one before it, three repetitions in that order and three in reverse. Medians of the six:

State	p50	p95	Client CPU	QPM CPU	Log lines per job
Off	10.91 ms	12.43 ms	7.51 ms	7.28 ms	0
Metrics (production default)	+0.13	12.29	+0.27	+0.17	0
Metrics and traces	+0.31	12.91	+0.47	+0.43	0
Plus the transport spans	+0.39	12.95	+0.58	+0.53	0
Plus logs at error	+0.49	13.16	+0.69	+0.68	0
Plus logs at debug (the overlay as shipped)	+0.67	13.87	+0.81	+1.33	10
Plus logs at all	+1.15	19.83	+0.87	+2.32	32

The two orders agree on every delta to within 0.1 ms of p50 (one state excepted) and 0.5 ms of CPU. The 2026-10-08 numbers for the first three states are within the spread of the repetitions.

What changed in the doc

• The table has the seven states and a column for the lines a job leaves in Loki.
• The prose explains the logs tier's cost: about 80 µs of QPM CPU per exported line, the p95 jump at all, and the plane's status polls that put about 16 lines a second into Loki at debug whatever the job rate (some 200 MB a day on an idle plane, which is what sets Loki's size over its retention).
• The budget gains a figure for the overlay as shipped: up to 1 ms of p50 and 3 ms of CPU per job. all is for a diagnosis, not a budget.
• The footprint counts Loki: the fifth container, 145 MB of image, the five containers' memory after two days of data, Tempo's size with the transport spans, and Loki's storage per line.