telemetry: record linking, the one expensive thing the server never mentioned

[oyvindberg]

A compile has compile_start/compile_end. A forked test JVM has five events joined on pid. A link had nothing — and that is the operation which forks a whole toolchain (for Scala Native: NIR → LLVM IR → clang → executable). #603 already charges it a cost against the machine governor, so it was accounted for and then invisible, and "why was this build slow" could not be answered for any build that links.

The events

link_start / link_end, carrying project, workspace, platform, release_mode, is_test, plus duration and success.

release_mode is included because it is the field that predicts the number — the same project linked Debug versus ReleaseFast differs by a large factor, and a total with both mixed in explains nothing. platform for the same reason across the four toolchains.

Emitted through a single withLinkMetrics helper rather than the same block copied into both link handlers. guaranteeCase rather than flatMap, for the reason the compile path uses it: a link that is cancelled or throws must still emit its end event, or the start dangles and the duration is unknowable — the bug shape fork_end exists to close.

Both readers, same commit — deliberately

#630 was the fix for these two drifting apart, and the dashboard silently dropping six of twenty event types via case _ => () is what that drift looked like. So:

• server-metrics gains a Linking card, red when a link failed.
• The CI summariser groups by platform and mode, so a Scala Native release link and a Scala.js debug link are not summed into one meaningless number.

Scope, stated plainly

This instruments the server's link path, which is what a real build uses. It does not measure the linking inside bleep-bsp-tests, where the suites drive the bridges directly inside a forked test JVM. That cost is already visible per suite via suite_finished — which is exactly how it was measured when those suites were tagged slow in #639.

Verification

Against synthetic events (Scala.js debug 3s, Scala Native release 90s, Scala Native test 14s failing):

  links: 3 (1 failed), total 1.8m
        1.5m  Scala Native release (1)
       14.0s  Scala Native (1)
        3.0s  Scala.js (1)

and the dashboard renders Linking — 3 links, 107.0s, 1 release-mode, 1 failed with the red accent. 107s = 3 + 90 + 14, so the arithmetic is right and the release-mode split lands where it should.

Full bleep compile green.

🤖 Generated with Claude Code