Capture Bigtable cluster zone, ID from trailers and include in traces

[dbolduc]

Bigtable returns a trailer: x-goog-ext-425905942-bin, which is the following proto encoded:

message RequestParams {
  optional string cluster_zone = 1;
  optional string cluster_id = 2;
}

This is the sort of information that would be good to capture in a trace.

This may prove difficult because tracing stubs are fully generated, and there is a custom scheme to interpret the binary trailer value.