Add Cloud Spanner OpenCensus integration samples

[c24t]

Following similar examples in other languages:

• https://github.com/GoogleCloudPlatform/java-docs-samples/pull/5437
• https://github.com/GoogleCloudPlatform/java-docs-samples/pull/5600
• https://github.com/GoogleCloudPlatform/golang-samples/pull/2163

cc @mayurkale22

[iennae]

should we be migrating to opentelemetry samples instead of OpenCensus?

[minherz]

+1 This should be either converted to OT example or closed. Also, I see that OT related examples are currently hosted at opentelemetry-operations-python repo. We should decide whether OT related examples should be delegated to the OT team or hosted here.