spanner: enable OpenCensus for tracing and metric collection
spanner: enable OpenCensus for tracing and metric collection

Manual root and child spans needs to be created and OpenCensus
exporters need to be registered to export the collected data.

OpenCensus stats views need to be registered by the user
in order to enable metric collection.

Change-Id: I5805925b9fe68839f988aedbb64e2fd1fce65fbd
Reviewed-on: https://code-review.googlesource.com/20130
Reviewed-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
Reviewed-by: Emmanuel Odeke <277f8eaa8c7dfbc55c17bf5e30561c1a9c45c569@gmail.com>
Reviewed-by: Jonathan Amsterdam <e3d3698b2ccd5955e4adf250d0785062d0f9018b@google.com>

googleapis/google-cloud-go,googleapis/google-cloud-go,googleapis/google-cloud-go,googleapis/google-cloud-go