Collector slabinfo (#6800)

### Summary

Provide new collector parsing `/proc/slabinfo` to provide details on kernel slab structures.

Asked by issue #13 (very happy for the oldest issue in backlog) 

##### Component Name

collectors/slabinfo.plugin

##### Additional Information

This slabinfo details allows to have clues on actions done on your system.
In the following screenshot, you can clearly see a `find` done on a ext4 filesystem (the number of `ext4_inode_cache` & `dentry` are rising fast), and a few seconds later, an admin issued a `echo 3 > /proc/sys/vm/drop_cached` as their count dropped.

1 files changed, 4 insertions, 1 deletions

diff --git a/docs/Add-more-charts-to-netdata.md b/docs/Add-more-charts-to-netdata.md
index 3b1bb96269..fe0341ceda 100644
--- a/docs/Add-more-charts-to-netdata.md
+++ b/docs/Add-more-charts-to-netdata.md
@@ -70,6 +70,7 @@ To control which plugins Netdata run, edit `netdata.conf` and check the `[plugin
 	# apps = yes
 	# xenstat = yes
 	# perf = no
+	# slabinfo = no
 ```
 
 The default for all plugins is the option `enable running new plugins`. So, setting this to `no` will disable all the plugins, except the ones specifically enabled.
@@ -96,7 +97,7 @@ sudo su -s /bin/bash netdata
 ```
 
 Similarly, you can use `charts.d.plugin` for BASH plugins and `node.d.plugin` for node.js plugins.
-Other plugins (like `apps.plugin`, `freeipmi.plugin`, `fping.plugin`, `ioping.plugin`, `nfacct.plugin`, `xenstat.plugin`, `perf.plugin`) use the native Netdata plugin API and can be run directly.
+Other plugins (like `apps.plugin`, `freeipmi.plugin`, `fping.plugin`, `ioping.plugin`, `nfacct.plugin`, `xenstat.plugin`, `perf.plugin`, `slabinfo.plugin`) use the native Netdata plugin API and can be run directly.
 
 If you need to configure a Netdata plugin or module, all user supplied configuration is kept at `/etc/netdata` while the stock versions of all files is at `/usr/lib/netdata/conf.d`.
 To copy a stock file and edit it, run `/etc/netdata/edit-config`. Running this command without an argument, will list the available stock files.
@@ -126,6 +127,7 @@ This is a map of the all supported configuration options:
 | `proc.plugin`<br/>(internal plugin for monitoring Linux system resources)|`C`|`netdata.conf` section `[plugin:proc]`|one section for each module `[plugin:proc:MODULE]`. Each module may provide additional sections in the form of `[plugin:proc:MODULE:SUBSECTION]`.|
 | `python.d.plugin`<br/>(external plugin orchestrator for running python modules)|`python`<br/>v2 or v3<br/>both are supported|`python.d.conf`|a file for each module in `/etc/netdata/python.d/`.|
 | `statsd.plugin`<br/>(internal plugin for collecting statsd metrics)|`C`|`netdata.conf` section `[statsd]`|Synthetic statsd charts can be configured with files in `/etc/netdata/statsd.d/`.|
+| `slabinfo.plugin`<br/>(external plugin for monitoring Kernel SLAB cache on Linux)|`C`|`netdata.conf` section `[plugin:slabinfo]`|N/A|
 | `tc.plugin`<br/>(internal plugin for collecting Linux traffic QoS)|`C`|`netdata.conf` section `[plugin:tc]`|The plugin runs an external helper called `tc-qos-helper.sh` to interface with the `tc` command. This helper supports a few additional options using `tc-qos-helper.conf`.|
 
 ## writing data collection modules
@@ -316,6 +318,7 @@ These are all the data collection plugins currently available.
 | cpu_apps|BASH<br/>Shell Script|Collects the CPU utilization of select apps.<br/><br/>DEPRECATED IN FAVOR OF `apps.plugin`. It is still supplied only as an example module to shell scripting plugins.<br/> <br/>Netdata plugin: [charts.d.plugin](../collectors/charts.d.plugin#chartsdplugin)<br/>plugin module: [cpu_apps.chart.sh](../collectors/charts.d.plugin/cpu_apps)<br/>configuration file: [charts.d/cpu_apps.conf](../collectors/charts.d.plugin/cpu_apps)|
 | load_average|BASH<br/>Shell Script|Collects the current system load average.<br/><br/>DEPRECATED IN FAVOR OF THE NETDATA INTERNAL ONE. It is still supplied only as an example module to shell scripting plugins.<br/> <br/>Netdata plugin: [charts.d.plugin](../collectors/charts.d.plugin#chartsdplugin)<br/>plugin module: [load_average.chart.sh](../collectors/charts.d.plugin/load_average)<br/>configuration file: [charts.d/load_average.conf](../collectors/charts.d.plugin/load_average)|
 | mem_apps|BASH<br/>Shell Script|Collects the memory footprint of select applications.<br/><br/>DEPRECATED IN FAVOR OF `apps.plugin`. It is still supplied only as an example module to shell scripting plugins.<br/> <br/>Netdata plugin: [charts.d.plugin](../collectors/charts.d.plugin#chartsdplugin)<br/>plugin module: [mem_apps.chart.sh](../collectors/charts.d.plugin/mem_apps)<br/>configuration file: [charts.d/mem_apps.conf](../collectors/charts.d.plugin/mem_apps)|
+| slabinfo|C|`slabinfo.plugin` collects Kernel SLAB cache metrics on Linux .<br/> <br/>[Documentation of `slabinfo.plugin`](../collectors/slabinfo.plugin/).<br/> <br/>Netdata plugin: [`slabinfo_plugin.c`](../collectors/slabinfo.plugin)|
 
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