Add Historic CPU and memory stats (#7136)

Collect historic cpu and mem stats.  Also, use actual values 
instead of formatted strings while returning to the client. The string 
formatting prevents values from being processed by the server or 
by the client without parsing it. 

This change will allow the values to be processed (eg. 
compute rolling-average over the lifetime of the minio server)
and offloads the formatting to the client.

pkg/mem/mem.go

@@ -18,22 +18,51 @@ package mem
 
 import (
 	"runtime"
-
-	humanize "github.com/dustin/go-humanize"
+	"time"
 )
 
+// historicUsage holds the rolling average of memory used by
+// minio server
+var historicUsage *Usage
+
+// memUsageMeasureInterval is the window of time between
+// two measurements of memory usage
+const memUsageMeasureInterval = 5 * time.Second
+
+// triggers the collection of historic stats about the memory
+// utilized by minio server
+func init() {
+	historicUsage = &Usage{}
+	var cycles uint64
+	go func() {
+		for {
+			time.Sleep(memUsageMeasureInterval)
+			currUsage := GetUsage()
+			currSum := cycles * historicUsage.Mem
+			cycles = cycles + 1
+			historicUsage.Mem = (currSum + currUsage.Mem) / cycles
+		}
+	}()
+}
+
 // Usage holds memory utilization information in human readable format
 type Usage struct {
-	Mem   string `json:"mem"`
+	Mem   uint64 `json:"mem"`
 	Error string `json:"error,omitempty"`
 }
 
+// GetHistoricUsage measures the historic average of memory utilized by
+// current process
+func GetHistoricUsage() Usage {
+	return *historicUsage
+}
+
 // GetUsage measures the total memory provisioned for the current process
 // from the OS
 func GetUsage() Usage {
 	memStats := new(runtime.MemStats)
 	runtime.ReadMemStats(memStats)
 	return Usage{
-		Mem: humanize.IBytes(memStats.Sys),
+		Mem: memStats.Sys,
 	}
 }