add btrfs and powersupplyclass to list of exporters enabled by default

Signed-off-by: Paul Manley <paul.manley@wholefoods.com>

README.md

@@ -29,6 +29,7 @@ Name     | Description | OS
 arp | Exposes ARP statistics from `/proc/net/arp`. | Linux
 bcache | Exposes bcache statistics from `/sys/fs/bcache/`. | Linux
 bonding | Exposes the number of configured and active slaves of Linux bonding interfaces. | Linux
+btrfs | Exposes btrfs statistics | Linux
 boottime | Exposes system boot time derived from the `kern.boottime` sysctl. | Darwin, Dragonfly, FreeBSD, NetBSD, OpenBSD, Solaris
 conntrack | Shows conntrack statistics (does nothing if no `/proc/sys/net/netfilter/` present). | Linux
 cpu | Exposes CPU statistics | Darwin, Dragonfly, FreeBSD, Linux, Solaris
@@ -50,6 +51,7 @@ netdev | Exposes network interface statistics such as bytes transferred. | Darwi
 netstat | Exposes network statistics from `/proc/net/netstat`. This is the same information as `netstat -s`. | Linux
 nfs | Exposes NFS client statistics from `/proc/net/rpc/nfs`. This is the same information as `nfsstat -c`. | Linux
 nfsd | Exposes NFS kernel server statistics from `/proc/net/rpc/nfsd`. This is the same information as `nfsstat -s`. | Linux
+powersupplyclass | Exposes Power Supply statistics from /sys/class/power_supply | Linux
 pressure | Exposes pressure stall statistics from `/proc/pressure/`. | Linux (kernel 4.20+ and/or [CONFIG\_PSI](https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/Documentation/accounting/psi.txt))
 rapl | Exposes various statistics from `/sys/class/powercap`. | Linux
 schedstat | Exposes task scheduler statistics from `/proc/schedstat`. | Linux