code: synchronized Spanner getSpanner(String projectId, Credentials credentials, String host) {
      SpannerKey key = SpannerKey.of(host, projectId, credentials);
      Spanner spanner = spanners.get(key);
      if (spanner == null) {
        spanner = createSpanner(key);
        spanners.put(key, spanner);
      }
      return spanner;
    }
docs: Get a {@link Spanner} instance from the pool or create a new one if needed.
  
  @param projectId The projectId to connect to
  @param credentials The credentials to use for the connection
  @param host The host to connect to. Normally this is https://spanner.googleapis.com, but you
  could also use a (local) emulator. If null, no host will be set and the default host of
  Google Cloud Spanner will be used.
  @return The {@link Spanner} instance to use