chore: include the build-duration-milliseconds github runner monitoring in the reusable workflow

[Kerruba]

Description

Centralize build-duration collection in the shared submit-build-status action

Previously repositories were calculating build_duration_millis in its own observe-build-status wrapper (duplicated logic, relying on an unreliable GITHUB_ACTION_PATH directory mtime). This moves the calculation into the shared start-build-monitor / submit-build-status actions.

What changed:

• start-build-monitor now records the job start time with millisecond precision (date +%s%3N) into a shared temp file, and clears any stale monitor state (PID/log/timestamp) at startup. It must remain the first job step so the duration covers the
  whole job.
• submit-build-status reads that timestamp when it stops the monitor, computes the elapsed duration, validates it (non-negative integer, capped at 72h / 259200000 ms), and submits it automatically. Invalid/missing/out-of-range timestamps omit the
  field with a warning instead of failing submission.
• Duration and resource/network metrics are now collected before Google auth and the BigQuery submission, so they exclude telemetry-submission overhead.
• Resolution precedence: explicit build_duration_millis input → monitor-derived duration → field omitted.
• Resource-metric computation (CPU/mem percentiles, network bytes) is unchanged.

Backward compatibility: Fully backward compatible.

• The optional build_duration_millis input is retained. Repositories that pass their own value override the automatic duration — an explicit non-empty input always takes precedence, so unmigrated wrappers keep working unchanged.
• Repositories that do not run start-build-monitor (or haven't migrated) simply get no automatic duration; the field is omitted (NULL in BigQuery), exactly as before.
• No consumer changes are required to adopt this; wrappers can drop their local duration calculation later, after this action ref is available.

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[Copilot]

Pull request overview

This PR centralizes build-duration collection in the shared start-build-monitor + submit-build-status actions, removing the need for per-repo duration calculation and ensuring duration/resource metrics are captured before auth/BigQuery submission.

Changes:

• Record job start time (epoch millis) in start-build-monitor and compute/validate elapsed duration in submit-build-status.
• Add explicit-duration validation and precedence resolution (explicit input overrides monitor-derived duration).
• Update submit-build-status workflow step ordering and documentation; add a shell test script for duration behavior.

Reviewed changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
submit-build-status/validate-build-duration.sh	Adds validation/normalization for explicit build_duration_millis input with max 72h cap.
submit-build-status/test_build_duration.sh	Adds a shell-based test harness covering duration validation and monitor-derived duration behavior.
submit-build-status/stop-and-collect-build-monitor.sh	Computes duration from start timestamp, makes monitor paths overridable, and emits build_duration_millis as a step output.
submit-build-status/resolve-build-duration.sh	Implements precedence selection between explicit and monitor-derived duration.
submit-build-status/README.md	Documents automatic duration behavior, precedence, and updated integration example.
submit-build-status/action.yml	Validates duration input, collects metrics earlier, resolves duration, and submits the resolved duration field.
start-build-monitor/start-build-monitor.sh	Writes start timestamp (epoch millis) and clears stale monitor state at startup; improves quoting.
start-build-monitor/README.md	Updates docs to include duration collection and guidance to keep the step first in the job.

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