storage: provide call context in the trace span

**Is your feature request related to a problem? Please describe.**

I'm trying to instrument the code that uses `cloud.google.com/go/storage` to access objects stored in Google Cloud Storage. Although the library already creates OpenCensus spans, the amount of information exposed about the call is insufficient for any troubleshooting.

**Describe the solution you'd like**

I'd like to propose adding more context to call spans, that would allow to distinguish between them. Such values as bucket name for bucket/object operations, the project ID would be very helpful in this regard. Besides tracking the start and end of method calls with `trace.StartSpan(ctx)`/`defer trace.EndSpan(ctx, err)` an additional call to `trace.TracePrintf()` with an attribute map carrying the context of a call would suffice.

**Describe alternatives you've considered**

My current alternative to this is wrapping every single entity that makes calls to the Storage API, creating a new span and attaching the call context. I'm happy to contribute and these changes to the upstream.