Add README for lustre2 input plugin

plugins/inputs/lustre2/README.md

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+# Lustre Input Plugin
+
+The [Lustre][]® file system is an open-source, parallel file system that supports
+many requirements of leadership class HPC simulation environments.
+
+This plugin monitors the Lustre file system using its entries in the proc filesystem.
+
+### Configuration
+
+```toml
+# Read metrics from local Lustre service on OST, MDS
+[[inputs.lustre2]]
+  ## An array of /proc globs to search for Lustre stats
+  ## If not specified, the default will work on Lustre 2.5.x
+  ##
+  # ost_procfiles = [
+  #   "/proc/fs/lustre/obdfilter/*/stats",
+  #   "/proc/fs/lustre/osd-ldiskfs/*/stats",
+  #   "/proc/fs/lustre/obdfilter/*/job_stats",
+  # ]
+  # mds_procfiles = [
+  #   "/proc/fs/lustre/mdt/*/md_stats",
+  #   "/proc/fs/lustre/mdt/*/job_stats",
+  # ]
+```
+
+### Metrics
+
+From `/proc/fs/lustre/obdfilter/*/stats` and `/proc/fs/lustre/osd-ldiskfs/*/stats`:
+
+- lustre2
+  - tags:
+    - name
+  - fields:
+    - write_bytes
+    - write_calls
+    - read_bytes
+    - read_calls
+    - cache_hit
+    - cache_miss
+    - cache_access
+
+From `/proc/fs/lustre/obdfilter/*/job_stats`:
+
+- lustre2
+  - tags:
+    - name
+    - jobid
+  - fields:
+    - jobstats_ost_getattr
+    - jobstats_ost_setattr
+    - jobstats_ost_sync
+    - jobstats_punch
+    - jobstats_destroy
+    - jobstats_create
+    - jobstats_ost_statfs
+    - jobstats_get_info
+    - jobstats_set_info
+    - jobstats_quotactl
+    - jobstats_read_bytes
+    - jobstats_read_calls
+    - jobstats_read_max_size
+    - jobstats_read_min_size
+    - jobstats_write_bytes
+    - jobstats_write_calls
+    - jobstats_write_max_size
+    - jobstats_write_min_size
+
+From `/proc/fs/lustre/mdt/*/md_stats`:
+
+- lustre2
+  - tags:
+    - name
+  - fields:
+    - open
+    - close
+    - mknod
+    - link
+    - unlink
+    - mkdir
+    - rmdir
+    - rename
+    - getattr
+    - setattr
+    - getxattr
+    - setxattr
+    - statfs
+    - sync
+    - samedir_rename
+    - crossdir_rename
+
+From `/proc/fs/lustre/mdt/*/job_stats`:
+
+- lustre2
+  - tags:
+    - name
+    - jobid
+  - fields:
+    - jobstats_close
+    - jobstats_crossdir_rename
+    - jobstats_getattr
+    - jobstats_getxattr
+    - jobstats_link
+    - jobstats_mkdir
+    - jobstats_mknod
+    - jobstats_open
+    - jobstats_rename
+    - jobstats_rmdir
+    - jobstats_samedir_rename
+    - jobstats_setattr
+    - jobstats_setxattr
+    - jobstats_statfs
+    - jobstats_sync
+    - jobstats_unlink
+
+
+### Troubleshooting
+
+Check for the default or custom procfiles in the proc filesystem, and reference
+the [Lustre Monitoring and Statistics Guide][guide].  This plugin does not
+report all information from these files, only a limited set of items
+corresponding to the above metric fields.
+
+### Example Output
+
+```
+lustre2,host=oss2,jobid=42990218,name=wrk-OST0041 jobstats_ost_setattr=0i,jobstats_ost_sync=0i,jobstats_punch=0i,jobstats_read_bytes=4096i,jobstats_read_calls=1i,jobstats_read_max_size=4096i,jobstats_read_min_size=4096i,jobstats_write_bytes=310206488i,jobstats_write_calls=7423i,jobstats_write_max_size=53048i,jobstats_write_min_size=8820i 1556525847000000000
+lustre2,host=mds1,jobid=42992017,name=wrk-MDT0000 jobstats_close=31798i,jobstats_crossdir_rename=0i,jobstats_getattr=34146i,jobstats_getxattr=15i,jobstats_link=0i,jobstats_mkdir=658i,jobstats_mknod=0i,jobstats_open=31797i,jobstats_rename=0i,jobstats_rmdir=0i,jobstats_samedir_rename=0i,jobstats_setattr=1788i,jobstats_setxattr=0i,jobstats_statfs=0i,jobstats_sync=0i,jobstats_unlink=0i 1556525828000000000
+
+```
+
+[lustre]: http://lustre.org/
+[guide]: http://wiki.lustre.org/Lustre_Monitoring_and_Statistics_Guide