Add perf exporter (#1274)

Signed-off-by: Daniel Hodges <hodges.daniel.scott@gmail.com>

README.md

@@ -63,6 +63,23 @@ zfs | Exposes [ZFS](http://open-zfs.org/) performance statistics. | [Linux](http
 
 ### Disabled by default
 
+The perf collector may not work by default on all Linux systems due to kernel
+configuration and security settings. To allow access, set the following sysctl
+parameter:
+
+```
+sysctl -w kernel.perf_event_paranoid=X
+```
+
+- 2 allow only user-space measurements (default since Linux 4.6).
+- 1 allow both kernel and user measurements (default before Linux 4.6).
+- 0 allow access to CPU-specific data but not raw tracepoint samples.
+- -1 no restrictions.
+
+Depending on the configured value different metrics will be available, for most
+cases `0` will provide the most complete set. For more information see [`man 2
+perf_event_open`](http://man7.org/linux/man-pages/man2/perf_event_open.2.html).
+
 Name     | Description | OS
 ---------|-------------|----
 buddyinfo | Exposes statistics of memory fragments as reported by /proc/buddyinfo. | Linux
@@ -81,6 +98,7 @@ supervisord | Exposes service status from [supervisord](http://supervisord.org/)
 systemd | Exposes service and system status from [systemd](http://www.freedesktop.org/wiki/Software/systemd/). | Linux
 tcpstat | Exposes TCP connection status information from `/proc/net/tcp` and `/proc/net/tcp6`. (Warning: the current version has potential performance issues in high load situations.) | Linux
 wifi | Exposes WiFi device and station statistics. | Linux
+perf | Exposes perf based metrics (Warning: Metrics are dependent on kernel configuration and settings). | Linux
 
 ### Textfile Collector