v3: improve metadata

[daniel-sanche]

metadata should include the following fields:

• x-goog-request-params
  • example: table_name=projects/my-project/instances/my-instance/tables/my-table&app_profile_id=my-app-profile
• bigtable-client-attempt-epoch-usec
  • Current time on the client machine in microseconds since utc epoch
• bigtable-attempt
  • Zero-based attempt count per operation
  • Java: https://github.com/googleapis/java-bigtable/blob/main/google-cloud-bigtable/src/main/java/com/google/cloud/bigtable/data/v2/stub/metrics/Util.java#L57-L60
• Cookie for tracing