Update `opentelemetry-proto` submodule to version from May 18, 2021 (#825)
Update `opentelemetry-proto` submodule to version from May 18, 2021 (#825)


open-telemetry/opentelemetry-cpp,open-telemetry/opentelemetry-cpp,open-telemetry/opentelemetry-cpp,open-telemetry/opentelemetry-cpp