docs: add :members: to OTLP exporter submodule automodule directives (#5124)

* docs: add :members: to OTLP exporter submodule automodule directives

The trace_exporter, metric_exporter, and _log_exporter entries in
docs/exporter/otlp/otlp.rst had no Sphinx autodoc options, so their
classes were not included in the generated API documentation.

Add :members:, :undoc-members:, and :show-inheritance: to each of the
six submodule automodule directives (three for proto.http, three for
proto.grpc) so the exporter classes appear on the rendered page.

Fixes #2786

Signed-off-by: Ali <alliasgher123@gmail.com>

* docs: remove changelog entry per review

Signed-off-by: Ali <alliasgher123@gmail.com>

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Signed-off-by: Ali <alliasgher123@gmail.com>
Co-authored-by: Mike Goldsmith <goldsmith.mike@gmail.com>