Add network metrics collection

[lzang]

Add the network metrics collection logic to collect the following metrics:

• conntrack entries: conntrack usage
• conntrack_error_count (insert_failed, and drop) : A request from TSE due to the issue mentioned here
  https://www.weave.works/blog/racy-conntrack-and-dns-lookup-timeouts
• num_inuse_sockets(tcp, udp): this would help to tell if the node is under some sort of ddos attack, or a particular node has an unusually high number of connections.
• num_tw_sockets: this can tell if there is a large number of short-lived sockets, which is often not desired.
• socket_memory: the memory used by all sockets. When the memory hits the limit, the network becomes not usable. There is a request to add this metrics (kubelet should track tcp_mem stats also along with cpu/ram/disk kubernetes/kubernetes#62334).
  All these metrics can be obtained by read proc file directly (i.e. O(1) cost) .

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CLAs look good, thanks!

[bowei]

Seems like my comment got eaten:

type conntrackStats struct {
  insertFailed int
  drop int
}

func (c *conntrackStats) merge(other *conntrackStats) {
  c.insertFailed += other.insertFailed
  c.drop += other.drop
}

type conntrackIndices struct {
  numFields int
  insert_failed_index int
  drop_index int
}

// parseHeader parses the conntrack header line, returning the
// indexes of the fields we wish to extract.
func parseHeader(line string) (conntrackIndicies, error) {
  var indicies conntrackIndicies
  nameParts = strings.Split(scanner.Text(), " ")
  indicies.numFields = len(nameParts)
  for i, v := range nameParts {
    switch v {
      case "insert_failed":
        indicies.insertFailed = i
      case "drop":
        indicies.drop = i
    }
  }
}

// parseConntrackData parses ...
func parseConntrackData(line string, indicies conntrackIndicies) (*conntrackStats, error) {
  stats := &conntrackStats{}
  valueParts := strings.Split(scanner.Text(), " ")
  if len(valueParts) != numFields { ... }

  if v, err := strconv.ParseUint(valueParts[indicies.insertFailed], 16, 32); err == nil {
    stats.insertFailed = v
  } else { ... }

  if v, err := strconv.ParseUint(valueParts[indicies.drops], 16, 32); err == nil {
    stats.drops = v
  } else { ... }

  return stats, nil
}

// parseConntrackFile parses the conntrack file contents read in from `r` and returns a merged set of stats.
func parseConntrackFile(r io.Reader) (conntrackStats, error) {
  ...
  if ! scanner.Scan() {
    // error no header line
  }
  indicies, err := parseHeader(scanner.Line())
  if err != nil { ... } 
  
  var accStats conntrackStats
  for scanner.Scan() {
     stats, err := parseConntrackData(scanner.Line(), indicies)
     if err != nil { ... }
     accStats.merge(stats)
  }
  ...
}

[bowei]

run golint and govet

minor comments