chore(api): enable request time histogram

[migmartri]

This patch enables an additional Prometheus histogram with the total time per request.

For example, upload times

grpc_server_handling_seconds_sum{grpc_method="Read",grpc_service="google.bytestream.ByteStream",grpc_type="server_stream"} 24.683958404000002
grpc_server_handling_seconds_count{grpc_method="Read",grpc_service="google.bytestream.ByteStream",grpc_type="server_stream"} 2

or client stream uploads

grpc_server_handling_seconds_sum{grpc_method="Write",grpc_service="google.bytestream.ByteStream",grpc_type="client_stream"} 10.418738306000002
grpc_server_handling_seconds_count{grpc_method="Write",grpc_service="google.bytestream.ByteStream",grpc_type="client_stream"} 4

This patch also tweaks the histogram buckets on the CAS case since those requests have different time baselines (uploads, download streaming)