Add a "kernel" plugin for /proc/stat statistics

see #235

plugins/inputs/system/KERNEL_README.md

@@ -0,0 +1,66 @@
+# Kernel Input Plugin
+
+This plugin is only available on Linux.
+
+The kernel plugin gathers info about the kernel that doesn't fit into other
+plugins. In general, it is the statistics available in `/proc/stat` that are
+not covered by other plugins.
+
+The metrics are documented in `man proc` under the `/proc/stat` section.
+
+```
+/proc/stat
+kernel/system statistics. Varies with architecture. Common entries include:
+
+page 5741 1808
+The number of pages the system paged in and the number that were paged out (from disk).
+
+swap 1 0
+The number of swap pages that have been brought in and out.
+
+intr 1462898
+This line shows counts of interrupts serviced since boot time, for each of
+the possible system interrupts. The first column is the total of all
+interrupts serviced; each subsequent column is the total for a particular interrupt.
+
+ctxt 115315
+The number of context switches that the system underwent.
+
+btime 769041601
+boot time, in seconds since the Epoch, 1970-01-01 00:00:00 +0000 (UTC).
+
+processes 86031
+Number of forks since boot.
+```
+
+### Configuration:
+
+```toml
+# Get kernel statistics from /proc/stat
+[[inputs.kernel]]
+  # no configuration
+```
+
+### Measurements & Fields:
+
+- kernel
+    - boot_time (integer, seconds since epoch, `btime`)
+    - context_switches (integer, `ctxt`)
+    - disk_pages_in (integer, `page (0)`)
+    - disk_pages_out (integer, `page (1)`)
+    - interrupts (integer, `intr`)
+    - processes_forked (integer, `processes`)
+    - swap_pages_in (integer, `swap (0)`)
+    - swap_pages_out (integer, `swap (1)`)
+
+### Tags:
+
+None
+
+### Example Output:
+
+```
+$ telegraf -config ~/ws/telegraf.conf -input-filter kernel -test
+* Plugin: kernel, Collection 1
+> kernel boot_time=1457505775i,context_switches=2626618i,disk_pages_in=5741i,disk_pages_out=1808i,interrupts=1472736i,processes_forked=10673i,swap_pages_in=1i,swap_pages_out=0i 1457613402960879816
+```

plugins/inputs/system/kernel.go

@@ -0,0 +1,122 @@
+package system
+
+// TODO add // +build linux
+
+import (
+	"bytes"
+	"fmt"
+	"io/ioutil"
+	"os"
+	"strconv"
+
+	"github.com/influxdata/telegraf"
+	"github.com/influxdata/telegraf/plugins/inputs"
+)
+
+// /proc/stat file line prefixes to gather stats on:
+var (
+	interrupts       = []byte("intr")
+	context_switches = []byte("ctxt")
+	processes_forked = []byte("processes")
+	disk_pages       = []byte("page")
+	swap_pages       = []byte("swap")
+	boot_time        = []byte("btime")
+)
+
+type Kernel struct {
+	statFile string
+}
+
+func (k *Kernel) Description() string {
+	return "Get kernel statistics from /proc/stat"
+}
+
+func (k *Kernel) SampleConfig() string { return "" }
+
+func (k *Kernel) Gather(acc telegraf.Accumulator) error {
+	data, err := k.getProcStat()
+	if err != nil {
+		return err
+	}
+
+	fields := make(map[string]interface{})
+
+	dataFields := bytes.Fields(data)
+	for i, field := range dataFields {
+		switch {
+		case bytes.Equal(field, interrupts):
+			m, err := strconv.Atoi(string(dataFields[i+1]))
+			if err != nil {
+				return err
+			}
+			fields["interrupts"] = m
+		case bytes.Equal(field, context_switches):
+			m, err := strconv.Atoi(string(dataFields[i+1]))
+			if err != nil {
+				return err
+			}
+			fields["context_switches"] = m
+		case bytes.Equal(field, processes_forked):
+			m, err := strconv.Atoi(string(dataFields[i+1]))
+			if err != nil {
+				return err
+			}
+			fields["processes_forked"] = m
+		case bytes.Equal(field, boot_time):
+			m, err := strconv.Atoi(string(dataFields[i+1]))
+			if err != nil {
+				return err
+			}
+			fields["boot_time"] = m
+		case bytes.Equal(field, disk_pages):
+			in, err := strconv.Atoi(string(dataFields[i+1]))
+			if err != nil {
+				return err
+			}
+			out, err := strconv.Atoi(string(dataFields[i+2]))
+			if err != nil {
+				return err
+			}
+			fields["disk_pages_in"] = in
+			fields["disk_pages_out"] = out
+		case bytes.Equal(field, swap_pages):
+			in, err := strconv.Atoi(string(dataFields[i+1]))
+			if err != nil {
+				return err
+			}
+			out, err := strconv.Atoi(string(dataFields[i+2]))
+			if err != nil {
+				return err
+			}
+			fields["swap_pages_in"] = in
+			fields["swap_pages_out"] = out
+		}
+	}
+
+	acc.AddFields("kernel", fields, map[string]string{})
+
+	return nil
+}
+
+func (k *Kernel) getProcStat() ([]byte, error) {
+	if _, err := os.Stat(k.statFile); os.IsNotExist(err) {
+		return nil, fmt.Errorf("kernel: %s does not exist!", k.statFile)
+	} else if err != nil {
+		return nil, err
+	}
+
+	data, err := ioutil.ReadFile(k.statFile)
+	if err != nil {
+		return nil, err
+	}
+
+	return data, nil
+}
+
+func init() {
+	inputs.Add("kernel", func() telegraf.Input {
+		return &Kernel{
+			statFile: "/proc/stat",
+		}
+	})
+}

plugins/inputs/system/kernel_test.go

@@ -0,0 +1 @@
+package system