Migrate to new OpenCensus APIs for OpenTelemetry migration

https://github.com/census-instrumentation/opencensus-java/pull/2059 added some new APIs to be able to use the gRPC context or the OpenTelemetry context. That change breaks gRPC (https://github.com/census-instrumentation/opencensus-java/issues/2069). Ignoring the breakage, we still want to figure out what changes we should make on our side and whether we can adopt the new API. We should also investigate what their migration looks like in the context of gRPC and its users.

Gribkoff, assigning to you for initial investigation, since this may influence your work. But I assume that before any larger changes we'd coordinate and determine who is appropriate.