Datastore tracing support

**Is your feature request related to a problem? Please describe.**
It would be great to have GCP Datastore tracing support in order to see all gRPC calls and how long do they take. It's possible to do with OpenCensus today but it doesn't work with Micrometer.

**Describe the solution you'd like**
GCP Datastore tracing is supported out-of-box

**Describe alternatives you've considered**
Wrap each datastore call with custom Span - it gives a little understanding on operations.

No files, tests, or entry points are named. Start by locating the project's GCP Datastore integration and its Micrometer tracing setup, then compare the requested behavior with the OpenCensus alternative described in the issue. Done means GCP Datastore tracing works out of the box and exposes gRPC calls with their durations.

Written by the indexing model from the issue text.