feat: subnet_metrics management canister method and million_round_instructions_total

[mraszyk]

Summary

Documents dfinity/ic#11587, which adds the million_round_instructions_total field to the subnet_metrics management canister endpoint.

The endpoint itself was added by dfinity/ic#11032 (merged 2026-09-02) and had never been documented here, so there was nothing for the new field to attach to. This PR therefore covers both: the method and all six of its result fields.

Interface specification

• New subnet_metrics method section in management-canister.md: canisters-only, EXPERIMENTAL, routed by the subnet_id in the argument so a canister can read another subnet's metrics, and free (no cycles, and serving it does not contribute to million_round_instructions_total).
• Per-field semantics and staleness: only block_height is current; the aggregates are as of the end of the previous round, and canister_state_bytes is only recomputed every 10 rounds.
• Which fields can be verified against a certificate: the four aggregates are the same values read_state serves under /subnet/<subnet_id>/metrics, while block_height and million_round_instructions_total have no path there.
• million_round_instructions_total is described as what it is: instructions in units of one million, rounded up, covering the scheduler's per-execution and per-canister overheads and non-Wasm charges (compilation, chunk assembly, snapshots), so not a Wasm instruction meter. Its counter starts when a subnet's replica begins tracking it, not at subnet creation.
• New transition rule in abstract-behavior.md, matching the shape of the node metrics rule (metrics values are implementation-specific).
• Pointer from the state tree's /subnet/<subnet_id>/metrics path in index.md to the method.
• public/references/ic.did: subnet_metrics_args, subnet_metrics_result, and the service method.
• Changelog entry 0.69.0.

Reference

• docs/references/management-canister.md: the method under "Subnet and node information", beside node_metrics_history and subnet_info.

Verification

• didc check public/references/ic.did passes.
• npm run build was not run: npm is unavailable in the environment this was written in. The validator's em-dash, forbidden-pattern, and internal link/anchor checks were run by hand over the five changed pages and are clean. CI is the gate.

🤖 Generated with Claude Code

[mraszyk]

Superseded: this duplicated the outstanding #333, which already specifies the subnet_metrics endpoint. The million_round_instructions_total field from dfinity/ic#11587 has been folded into #333 instead.