Rethink how GCS+gRPC is built

We are taking a hard dependency on `opentelemetry-cpp` and a soft dependency on gRPC's OTel plugin, even though both are required for metrics. It seems like we would want either both deps to be optional, or neither to be optional.

CMake and Bazel are also out of step.

We need to make progress though, so let's come back to this decision later.