EnableTrace in Plugin-cfg.xml

WebServer Plugin writes a log, by default its named as http-plugin.log, by default placed under PLUGIN_HOME/logs/
Plugin writes Error messages into this log. The attribute which deals with this is
< Log > in the plugin-cfg.xml
Ex.,
< Log LogLevel="Error" Name="/usr/IBM/WebSphere/Plugins/logs/http_plugin.log" / >

According to above line all Error messages will be written into http-plugin.log.

How to enable trace in the plugin-cfg.xml? if that is the question, do like this -

< Log LogLevel="Trace" Name="/usr/IBM/WebSphere/Plugins/logs/http_plugin.log" / >

From the InfoCenter -
Plug-in Problem Determination Steps
The plug-in provides very readable tracing which can be beneficial in helping to figure out the problem. By setting the LogLevel attribute in the config/plugin-cfg.xml file to Trace, you can follow the request processing to see what is going wrong.
Note: If you are using a Veritas File System with large file support enabled, file sizes up to two terabytes are allowed. In this case, if you set the LogLevel attribute in the plugin-cfg.xml file to LogLevel=Trace, then the http_plugin.log file might grow quickly and consume all available space on your file system. Therefore, you should set the value of the LogLevel attribute to ERROR or DEBUG to prevent high CPU utilization.
At a high level, complete these steps.
The plug-in gets a request.
The plug-in checks the routes defined in the plugin-cfg.xml file.
It finds the server group.
It finds the server.
It picks the transport protocol, HTTP or HTTPS.
It sends the request.
It reads the response.
It writes it back to the client.