Add support for gathering Slab memory usage

This is useful, for example when monitoring Slab memory leaks. To support
gathering this metric a minimum version of psutil 5.4.4 is required
(released on Apr 13th 2018).

Story: 2006815
Task: 37375
Change-Id: Ibe8def9e2a7c967a34236889aa03b287065abcdc

docs/Plugins.md

@@ -459,6 +459,7 @@ instances:
 | mem.used_cached |  | Mbytes of memory used for the page cache
 | mem.used_shared  |  | Mbytes of memory shared between separate processes and typically used for inter-process communication
 | mem.used_real_mb |  | Mbytes of memory currently in use less mem.used_buffers and mem.used_cached
+| mem.used_slab_mb  |  | Mbytes of memory currently allocated to slab. Requires psutil >=5.4.4.
 
 ### Disk
 | Metric Name | Dimensions | Semantics |

monasca_agent/collector/checks_d/memory.py

@@ -97,4 +97,11 @@ class Memory(checks.AgentCheck):
                        dimensions=dimensions)
             count += 1
 
+        # The slab metric was added in psutil 5.4.4
+        if hasattr(mem_info, 'slab') and mem_info.slab:
+            self.gauge('mem.used_slab_mb',
+                       int(mem_info.slab / 1048576),
+                       dimensions=dimensions)
+            count += 1
+
         log.debug('Collected {0} memory metrics'.format(count))

releasenotes/notes/add-slab-usage-metric-e7504c57e4b3ba16.yaml

@@ -0,0 +1,3 @@
+---
+features:
+  - Adds support for monitoring slab memory usage.