docs: add background info for session pool (#2498)

* docs: add background info for session pool

Adds additional technical background information on how the session and
and channel pools work.

* 🦉 Updates from OwlBot post-processor

See https://github.com/googleapis/repo-automation-bots/blob/main/packages/owl-bot/README.md

* chore: add code formatting

---------

Co-authored-by: Owl Bot <gcf-owl-bot[bot]@users.noreply.github.com>

.readme-partials.yaml

@@ -41,6 +41,15 @@ custom_content: |
 
   In [DatabaseSelect.java](https://github.com/googleapis/google-cloud-java/tree/master/google-cloud-examples/src/main/java/com/google/cloud/examples/spanner/snippets/DatabaseSelect.java) we put together all the code shown above in a single program.
 
+  ## Session Pool
+
+  The Cloud Spanner client maintains a session pool, as sessions are expensive to create and are
+  intended to be long-lived. The client automatically takes a session from the pool and uses this
+  executing queries and transactions.
+  See [Session Pool and Channel Pool Configuration](session-and-channel-pool-configuration.md)
+  for in-depth background information about sessions and gRPC channels and how these are handled in
+  the Cloud Spanner Java client.
+
   ## OpenCensus Metrics
 
   Cloud Spanner client supports [Opencensus Metrics](https://opencensus.io/stats/),

README.md

@@ -147,6 +147,15 @@ try {
 
 In [DatabaseSelect.java](https://github.com/googleapis/google-cloud-java/tree/master/google-cloud-examples/src/main/java/com/google/cloud/examples/spanner/snippets/DatabaseSelect.java) we put together all the code shown above in a single program.
 
+## Session Pool
+
+The Cloud Spanner client maintains a session pool, as sessions are expensive to create and are
+intended to be long-lived. The client automatically takes a session from the pool and uses this
+executing queries and transactions.
+See [Session Pool and Channel Pool Configuration](session-and-channel-pool-configuration.md)
+for in-depth background information about sessions and gRPC channels and how these are handled in
+the Cloud Spanner Java client.
+
 ## OpenCensus Metrics
 
 Cloud Spanner client supports [Opencensus Metrics](https://opencensus.io/stats/),