oss_28_fix_sphinx_docs_build

[ocelotl]

Closes #39

Fix the broken Sphinx -W docs build (Linear OSS-28, D1/D2): force pure-Python protobuf in docs/conf.py so the zipkin generated pb2 module autodocs under modern protobuf (D1); add opentelemetry-exporter-opencensus to docs-requirements + document its trace_exporter submodule + widen its protobuf constraint (>=3.13,<8.0) so the docs install resolves (D2).

Validation: verified via a real sphinx-build -E -a -W run — exits 0; zipkin proto module documented; opencensus page renders 6 objects (was 0). Scope: docs/ + docs-requirements + opencensus pyproject + changelog.

Linear issue: https://linear.app/dash0/issue/OSS-28/d1-d2-fix-broken-sphinx-docs-build-zipkin-protobuf-conflict-opencensus

[ocelotl]

🔒 Internal (dash0) — not for upstream.

• Linear: https://linear.app/dash0/issue/OSS-28/d1-d2-fix-broken-sphinx-docs-build-zipkin-protobuf-conflict-opencensus (D1/D2)
• Closes: oss_28_fix_sphinx_docs_build #39 · Branch: oss_28_fix_sphinx_docs_build · Commit: 9716beb1a
• Validation: sphinx-build -E -a -W exit 0 (zipkin documented; opencensus page 6 objects). Scope docs + docs-requirements + opencensus pyproject + changelog.

[ocelotl]

📣 Public-facing draft — to be used for the upstream PR in open-telemetry/opentelemetry-python. No internal references; copy verbatim.

Title: Fix broken Sphinx docs build for zipkin proto and opencensus exporters

What

Fix the documentation build so sphinx-build -W succeeds, the zipkin-proto-http exporter is documented, and the OpenCensus exporter page renders its API.

Why

• The zipkin-proto-http generated zipkin_pb2.py (old descriptor style) raises Descriptors cannot be created directly under the modern protobuf runtime installed for docs, so autodoc drops the module and the -W build fails. The tox docs env works around this with an env var, but the Read the Docs build (which calls sphinx directly) does not.
• The OpenCensus exporter doc page rendered zero objects: the package was never installed for docs (only the shim was), and the rst referenced the empty top-level package rather than the trace_exporter submodule that holds OpenCensusSpanExporter.

How

• Set PROTOCOL_BUFFERS_PYTHON_IMPLEMENTATION=python in docs/conf.py via os.environ.setdefault, so the fix applies regardless of how sphinx is invoked (tox and RTD).
• Add opentelemetry-exporter-opencensus to docs-requirements.txt and an automodule for its trace_exporter submodule in the rst.
• Widen the opencensus exporter's protobuf constraint from ~= 3.13 to >= 3.13, < 8.0; the old pin conflicts with opentelemetry-proto (>= 5.0) and made the docs install unresolvable. The exporter imports/runs under protobuf 5.

How verified

Ran sphinx-build -E -a -W -b html -T with the external env var unset — build succeeds (exit 0). The zipkin proto exporter is documented, and the OpenCensus page renders its objects including OpenCensusSpanExporter (previously 0). Confirmed pip install -r docs-requirements.txt resolves with all changes.