Activate filesystem collector on DragonFly (#302)

README.md

@@ -23,7 +23,7 @@ cpu | Exposes CPU statistics | FreeBSD
 diskstats | Exposes disk I/O statistics from `/proc/diskstats`. | Linux
 entropy | Exposes available entropy. | Linux
 filefd | Exposes file descriptor statistics from `/proc/sys/fs/file-nr`. | Linux
-filesystem | Exposes filesystem statistics, such as disk space used. | FreeBSD, Linux, OpenBSD
+filesystem | Exposes filesystem statistics, such as disk space used. | FreeBSD, Dragonfly, Linux, OpenBSD
 loadavg | Exposes load average. | Darwin, Dragonfly, FreeBSD, Linux, NetBSD, OpenBSD, Solaris
 mdadm | Exposes statistics about devices in `/proc/mdstat` (does nothing if no `/proc/mdstat` present). | Linux
 meminfo | Exposes memory statistics. | FreeBSD, Linux

collector/filesystem_bsd.go

@@ -11,7 +11,7 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
-// +build freebsd openbsd darwin,amd64
+// +build freebsd openbsd darwin,amd64 dragonfly
 // +build !nofilesystem
 
 package collector

collector/filesystem_common.go

@@ -12,7 +12,7 @@
 // limitations under the License.
 
 // +build !nofilesystem
-// +build linux freebsd openbsd darwin,amd64
+// +build linux freebsd openbsd darwin,amd64 dragonfly
 
 package collector