chore: switch to SPDX license headers

[MikeGoldsmith]

Summary

• Replace the 13-line Apache 2.0 license header with the 2-line SPDX equivalent (# SPDX-License-Identifier: Apache-2.0) across all Python files
• Add scripts/check_license_header.py enforcement script and lint-license-header-check tox env to catch missing headers in CI
• Document the required header format in CONTRIBUTING.md

541 files had their existing Apache header replaced with the SPDX equivalent. During the migration, 31 files were found with missing or inconsistent headers:

• 15 source/test files had no header at all (e.g. opentelemetry-api/src/opentelemetry/trace/span.py, files in tests/events/, tests/metrics/, shim/tests/testbed/)
• 4 exporter files had a compact Apache header variant (missing the blank # line after the copyright)
• 12 scripts and docs/examples files had no header

All of these now have the correct SPDX header. Generated files (*_pb2.py, *_pb2_grpc.py, datamodel-codegen output) and empty __init__.py files are excluded from the check.

Motivation

Other OTel SDKs have already adopted SPDX headers:

• Go: Already uses SPDX format
• Java: Already uses SPDX format

The SPDX format is functionally and legally equivalent. It reduces visual noise and aligns Python with the rest of the OTel ecosystem.

Closes #5170

Changes

Area	Detail
541 files	Replaced standard 13-line Apache header with 2-line SPDX
4 files	Fixed compact Apache header variant (missing blank comment line)
15 files	Added SPDX header to source/test files that never had one
12 files	Added SPDX header to scripts and docs/examples
CI	New lint-license-header-check tox env + regenerated lint.yml
Docs	Updated CONTRIBUTING.md Style Guide section

[herin049]

Small nit, but you could make this a bit less verbose:

Suggested change

	root = os.path.dirname(os.path.dirname(os.path.abspath(__file__)))
	root = str(Path(__file__).resolve().parents[1])