Provide trace instrumentation for Cloud SQL

**Is your feature request related to a problem? Please describe.**
Cloud SQL offers traces and a way of [connecting them to existing traces by using SQL comments](https://cloud.google.com/sql/docs/postgres/using-query-insights#adding-tags-to-sql-queries). There is currently no way of doing this that works for any Java libraries except for Hibernate.

**Describe the solution you'd like**
Integration trace instrumentation support into spring-cloud-gcp Cloud SQL so this works out of the box. Just like this is being offered for [Pub/Sub](https://googlecloudplatform.github.io/spring-cloud-gcp/5.4.3/reference/html/index.html#pubsub-trace-instrumentation-experimental). Applying this on the datasource layer could provide a universal solution.

**Describe alternatives you've considered**
We've looked at [sqlcommenter](https://google.github.io/sqlcommenter/) but this only works for Hibernate.

Start by inspecting the Cloud SQL datasource layer and the existing Pub/Sub trace instrumentation described in the linked reference. Define the datasource-level integration so SQL comments connect Cloud SQL queries to existing traces, and verify that it works for Java libraries beyond Hibernate.

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