[ymotongpoo]

The standalone CLI already creates OpenTelemetry spans internally, but it has no SDK/exporter initialization. This adds opt-in OTLP export so users can inspect those spans without modifying the CLI.

Closes #2609.

Behavior

• OTEL_TRACES_EXPORTER=otlp enables a command root span and the existing internal spans. Export stays disabled by default; OTEL_SDK_DISABLED=true also skips initialization.
• Support gRPC (default) and HTTP/protobuf, including trace-specific protocol overrides and the exporters' standard endpoint, header, timeout, and TLS environment variables.
• Preserve the original context, provider, and command result when tracing is disabled. Keep initialization in the executable; library callers retain their own SDK configuration.
• End the root span and allow up to five seconds to flush after success, errors, or cancellation. Report telemetry failures separately without replacing or wrapping the command error.

Validation

• go test ./..., make test (all packages with the race detector), and golangci-lint 2.14 passed.
• make generate left tracked generated files unchanged; go mod tidy completed.
• Compared --help, version, and a missing-config build against unmodified main. With VCS stamping disabled for comparison, stdout, stderr, and exit codes match.
• Built an image containing a locally packaged GNU Hello. It printed Hello, world!; Tempo returned 130 spans with one root and complete recorded parent relationships.
• Local OTLP receivers test both transports, resource attributes, existing child spans, command failures, cancellation, and exporter failures. Removing the opt-in guard makes the tests fail with disabled tracing changed the command context.

Scope

Existing build logic and internal span placement stay unchanged. Propagating context between separate CLI invocations remains a separate proposal. This implements the compatibility policy discussed in the issue; transport and configuration choices remain open to maintainer feedback.