Prior to this commit, if the TaskExecutor configured in
WebSocketMessageBrokerStats for the inboundChannelExecutor or
outboundChannelExecutor was not a ThreadPoolTaskExecutor, a
StringIndexOutOfBoundsException was thrown when attempting to parse the
results of invoking toString() on the executor.
The reason is that ThreadPoolTaskExecutor delegates to a
ThreadPoolExecutor whose toString() implementation generates text
containing "pool size = ...", and WebSocketMessageBrokerStats'
getExecutorStatsInfo() method relied on the presence of "pool" in the
text returned from toString().
This commit fixes this bug by ensuring that the text returned from
toString() contains "pool" before parsing the text. If "pool" is not
present in the text, getExecutorStatsInfo() now returns "unknown"
instead of throwing a StringIndexOutOfBoundsException.
Closes gh-27209
f56d6ea1fb