Add dmcache input plugin (#1667)

plugins/inputs/dmcache/README.md

@@ -0,0 +1,47 @@
+# DMCache Input Plugin
+
+This plugin provide a native collection for dmsetup based statistics for dm-cache.
+
+This plugin requires sudo, that is why you should setup and be sure that the telegraf is able to execute sudo without a password.
+
+`sudo /sbin/dmsetup status --target cache` is the full command that telegraf will run for debugging purposes.
+
+### Configuration
+
+```toml
+[[inputs.dmcache]]
+  ## Whether to report per-device stats or not
+  per_device = true
+```
+
+### Measurements & Fields:
+
+- dmcache
+    - length
+    - target
+    - metadata_blocksize
+    - metadata_used
+    - metadata_total
+    - cache_blocksize
+    - cache_used
+    - cache_total
+    - read_hits
+    - read_misses
+    - write_hits
+    - write_misses
+    - demotions
+    - promotions
+    - dirty
+
+### Tags:
+
+- All measurements have the following tags:
+    - device
+
+### Example Output:
+
+```
+$ ./telegraf --test --config /etc/telegraf/telegraf.conf --input-filter dmcache
+* Plugin: inputs.dmcache, Collection 1
+> dmcache,device=example cache_blocksize=0i,read_hits=995134034411520i,read_misses=916807089127424i,write_hits=195107267543040i,metadata_used=12861440i,write_misses=563725346013184i,promotions=3265223720960i,dirty=0i,metadata_blocksize=0i,cache_used=1099511627776ii,cache_total=0i,length=0i,metadata_total=1073741824i,demotions=3265223720960i 1491482035000000000
+```