Collect slab memory usage also in Linux.

collector/system_linux.go

@@ -123,8 +123,10 @@ func (c *collector) getMemory() {
 	c.result.System.MemFree = memInfo["MemFree:"]
 	c.result.System.MemBuffers = memInfo["Buffers:"]
 	c.result.System.MemCached = memInfo["Cached:"]
+	c.result.System.MemSlab = memInfo["Slab:"]
 	c.result.System.MemUsed = memInfo["MemTotal:"] - c.result.System.MemFree -
-		c.result.System.MemBuffers - c.result.System.MemCached
+		c.result.System.MemBuffers - c.result.System.MemCached -
+		c.result.System.MemSlab
 
 	// Swap
 	if val, ok := memInfo["SwapTotal:"]; ok {

model.go

@@ -19,7 +19,7 @@ package pgmetrics
 // ModelSchemaVersion is the schema version of the "Model" data structure
 // defined below. It is in the "semver" notation. Version history:
 //    1.8 - AWS RDS/EnhancedMonitoring metrics, index defn,
-//				backend type counts, user agent
+//				backend type counts, slab memory (linux), user agent
 //    1.7 - query execution plans, autovacuum, deadlocks, table acl
 //    1.6 - added highest WAL segment number
 //    1.5 - add PID to replication_outgoing entries
@@ -224,6 +224,8 @@ type SystemMetrics struct {
 	SwapUsed   int64   `json:"swapused"`            // used swap memory in bytes, 0 if no swap
 	SwapFree   int64   `json:"swapfree"`            // free swap memory in bytes, 0 if no swap
 	Hostname   string  `json:"hostname"`            // hostname from the OS
+	// following fields present only in schema 1.8 and later
+	MemSlab int64 `json:"memslab"` // RAM used for slab in bytes
 }
 
 type Backend struct {