dashboard: CPU, memory, diskIO and traffic on the footer (#15950)

* dashboard: footer, deep state update

* dashboard: resolve asset path

* dashboard: prevent state update on every reconnection

* dashboard: fix linter issue

* dashboard, cmd: minor UI fix, include commit hash

* dashboard: gitCommit renamed to commit

* dashboard: move the geth version to the right, make commit optional

* dashboard: memory, traffic and CPU on footer

* dashboard: fix merge

* dashboard: CPU, diskIO on footer

* dashboard: rename variables, use group declaration

* dashboard: docs

vendor/github.com/elastic/gosigar/concrete_sigar.go

@@ -0,0 +1,83 @@
+package gosigar
+
+import (
+	"time"
+)
+
+type ConcreteSigar struct{}
+
+func (c *ConcreteSigar) CollectCpuStats(collectionInterval time.Duration) (<-chan Cpu, chan<- struct{}) {
+	// samplesCh is buffered to 1 value to immediately return first CPU sample
+	samplesCh := make(chan Cpu, 1)
+
+	stopCh := make(chan struct{})
+
+	go func() {
+		var cpuUsage Cpu
+
+		// Immediately provide non-delta value.
+		// samplesCh is buffered to 1 value, so it will not block.
+		cpuUsage.Get()
+		samplesCh <- cpuUsage
+
+		ticker := time.NewTicker(collectionInterval)
+
+		for {
+			select {
+			case <-ticker.C:
+				previousCpuUsage := cpuUsage
+
+				cpuUsage.Get()
+
+				select {
+				case samplesCh <- cpuUsage.Delta(previousCpuUsage):
+				default:
+					// Include default to avoid channel blocking
+				}
+
+			case <-stopCh:
+				return
+			}
+		}
+	}()
+
+	return samplesCh, stopCh
+}
+
+func (c *ConcreteSigar) GetLoadAverage() (LoadAverage, error) {
+	l := LoadAverage{}
+	err := l.Get()
+	return l, err
+}
+
+func (c *ConcreteSigar) GetMem() (Mem, error) {
+	m := Mem{}
+	err := m.Get()
+	return m, err
+}
+
+func (c *ConcreteSigar) GetSwap() (Swap, error) {
+	s := Swap{}
+	err := s.Get()
+	return s, err
+}
+
+func (c *ConcreteSigar) GetFileSystemUsage(path string) (FileSystemUsage, error) {
+	f := FileSystemUsage{}
+	err := f.Get(path)
+	return f, err
+}
+
+func (c *ConcreteSigar) GetFDUsage() (FDUsage, error) {
+	fd := FDUsage{}
+	err := fd.Get()
+	return fd, err
+}
+
+// GetRusage return the resource usage of the process
+// Possible params: 0 = RUSAGE_SELF, 1 = RUSAGE_CHILDREN, 2 = RUSAGE_THREAD
+func (c *ConcreteSigar) GetRusage(who int) (Rusage, error) {
+	r := Rusage{}
+	err := r.Get(who)
+	return r, err
+}