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The following untested change to /etc/tomcat6/server.xml should turn on compression for JSON responses starting at 2KiB. (I took the opportunity to also changes "metrics" to "onionoo" in two places.)

                maxThreads="150" minSpareThreads="25" maxSpareThreads="75"
                enableLookups="false" redirectPort="8443" acceptCount="100"
                connectionTimeout="20000" disableUploadTimeout="true"
-               compression="off" compressionMinSize="2048"
+               compression="on" compressionMinSize="2048"
                noCompressionUserAgents="gozilla, traviata"
-               compressableMimeType="text/html,text/xml,text/plain" />
+               compressableMimeType="application/json" />
     <Engine name="Catalina" defaultHost="sewerzowi.torproject.org">
-      <Host name="metrics.torproject.org" appBase="webapps"
+      <Host name="onionoo.torproject.org" appBase="webapps"
             unpackWARs="true" autoDeploy="true"
             xmlValidation="false" xmlNamespaceAware="false">
         <Alias>sewerzowi.torproject.org</Alias>
         <Valve className="org.apache.catalina.valves.AccessLogValve"
-               directory="logs" prefix="metrics_access_log." suffix=".txt"
+               directory="logs" prefix="onionoo_access_log." suffix=".txt"
                pattern="%l %u %t %r %s %b" resolveHosts="false"/>
       </Host>
     </Engine>

If It's easier to compress responses in Apache, not in Tomcat, that's fine by me, too.