feat: Updating readme with new gfe latency metrics (#1630)

* feat: Updating readme with new gfe latency metrics * Updating readme with latest opencensus version and updating metrics list

Author: KiranmayiB

.readme-partials.yaml

@@ -67,9 +67,9 @@ custom_content: |
  * `cloud.google.com/java/spanner/max_allowed_sessions`: This shows the maximum
  number of sessions allowed.
 
- * `cloud.google.com/java/spanner/in_use_sessions`: This metric allows users to
+ * `cloud.google.com/java/spanner/num_sessions_in_pool`: This metric allows users to
  see instance-level and database-level data for the total number of sessions in
- use (or checked out from the pool) at this very moment.
+ the pool at this very moment.
 
  * `cloud.google.com/java/spanner/num_acquired_sessions`: This metric allows
  users to see the total number of acquired sessions.
@@ -83,24 +83,31 @@ custom_content: |
  it waits until a session is released into the pool by another thread) due to
  pool exhaustion since the server process started.
 
+ * `cloud.google.com/java/spanner/gfe_latency`: This metric shows latency between
+ Google's network receiving an RPC and reading back the first byte of the response.
+
+ * `cloud.google.com/java/spanner/gfe_header_missing_count`: This metric shows the
+ number of RPC responses received without the server-timing header, most likely
+ indicating that the RPC never reached Google's network.
+
  If you are using Maven, add this to your pom.xml file
  ```xml
  <dependency>
  <groupId>io.opencensus</groupId>
  <artifactId>opencensus-impl</artifactId>
- <version>0.26.0</version>
+ <version>0.30.0</version>
  <scope>runtime</scope>
  </dependency>
  <dependency>
  <groupId>io.opencensus</groupId>
  <artifactId>opencensus-exporter-stats-stackdriver</artifactId>
- <version>0.26.0</version>
+ <version>0.30.0</version>
  </dependency>
  ```
  If you are using Gradle, add this to your dependencies
  ```Groovy
- compile 'io.opencensus:opencensus-impl:0.26.0'
- compile 'io.opencensus:opencensus-exporter-stats-stackdriver:0.26.0'
+ compile 'io.opencensus:opencensus-impl:0.30.0'
+ compile 'io.opencensus:opencensus-exporter-stats-stackdriver:0.30.0'
  ```
 
  At the start of your application configure the exporter: