fix: Add automatic tracing to CLI subcommands

[ss1909]

Description

When BUILDKITE_TRACING_BACKEND=opentelemetry is configured, the bootstrap process initialises a TracerProvider and creates spans for each job phase (artifact-upload, etc.). However, subcommands like buildkite-agent artifact upload run as separate OS processes and never initialised their own TracerProvider, so even though TRACEPARENT and BUILDKITE_TRACING_BACKEND were present in their environment, no spans were emitted. The executor's phase spans were not visible internally.

Context

https://linear.app/buildkite/issue/A-1168/add-automatic-tracing-to-cli-subcommands-artifact-upload-pipeline

Changes

• internal/job/tracing.go — extracted InitOTelTracerProvider as a standalone exported function; startTracingOpenTelemetry now delegates to it

• clicommand/global.go — setupLoggerAndConfig now initialises a TracerProvider and extracts TRACEPARENT/TRACESTATE into context when running as a subprocess of a traced job

Every subcommand (artifact upload, pipeline upload, etc.) gets tracing for free.

• ctx, span := otel.Tracer("buildkite-agent").Start(ctx, "<sub-command-name>")
  create a new span named after the sub-command-name. Because ctx already has the parent trace context extracted from TRACEPARENT, OTel automatically makes this span a child of the executor's span.

Testing

• Tests have run locally (with go test ./...). Buildkite employees may check this if the pipeline has run automatically.
• Code is formatted (with go tool gofumpt -extra -w .)

Grafana screenshots:

Disclosures / Credits

Implementation was carried out in consultation with Claude

[buildkate]

small nit in comments, but otherwise approved