maximum length of item key should be increased

There are certain monitoring needs where the current limitation of 255 characters is not enough. For instance, monitoring WebSphere Application Server, where item keys take the following form and can be much longer than the current limit:

jmx["WebSphere:name=ibmasyncrsp#ibmasyncrsp.war,process=server1,platform=dynamicproxy,node=companyNode01,version=8.5.5.0,type=SessionManager,mbeanIdentifier=default_host/IBM_WS_SYS_RESPONSESERVLET,cell=companyNode01Cell,spec=1.0","name"]

This obviously raises the question of usability, like how to show keys in the frontend that are this long. However, at this point, before releasing 2.2, we should just increase the limit and address usability issues in 2.2.x.