How do I monitor the latest log file which the filename changes regularly?

There are two ways to design the FKM to look in the latest file, these are:

First, you may just input a wildcard at the end of siteserver. E.g.

FKM plugin supports using * and ? characters, as typically used in a UNIX shell session.

This will monitor the siteserver*.log file that has the latest timestamp. FKM's Advanced tab has an "Wildcard Match Time" option to control how the "latest" file is picked.

changed (default): In Unix, the changed time is updated when a file inode is updated, which occurs when a file is created, moved, copied, or the file permissions are changed. This is the default FKM setting since most processes monitored by FKM tend to roll-over log files by creating a new file and using this for logging. In addition some backup utilities alter the modification time during backup, which can interfere with monitoring if this time is checked. Note: The value of changed time for Windows and Unix is not the same. In Windows, the changed time is the same with creation time. It does not change even if the status of the file changes. modified: The modified time is updated when the content of a file is changed. This time may be preferred over the changed time if monitored log files will be renamed or their permissions changed.

In addition, when using the wildcard approach, kindly ensure that the setting Wildcard monitor all matches is not ticked (unchecked) or greyed out. If the said setting is ticked (checked), FKM will monitor all files that starts with siteserver and ends with .log. The said setting can also be found in the FKM sampler's Advanced tab.

Second, if your today file contains today's date, then you can use the today function. E.g. 

This will monitor the file with filename format siteserverYYYY-MM-DD.log

For the filename date and time format supported in FKM plugin, users can refer to the Date Generation section in the documentation.