Extra format argument to String.format call in SessionPool

[cushon]

This code is passing an argument to String.format that is ignored, because the string doesn't have any format specifiers:

java-spanner/google-cloud-spanner/src/main/java/com/google/cloud/spanner/SessionPool.java

Lines 3664 to 3669 in 6f522dd

	logger.log(
	Level.WARNING,
	String.format(
	"Failed to create multiplexed session. "
	+ "Pending replacing stale multiplexed session",
	t));

The intent may have been:

 logger.log( 
     Level.WARNING, 
     "Failed to create multiplexed session. " 
         + "Pending replacing stale multiplexed session", 
     t); 

This bug was caught by https://errorprone.info/bugpattern/FormatString