Add qdisc collector for Linux (#580)

* Add qdisc collector for Linux

This collector gathers basic queueing discipline metrics via netlink,
similarly to what `tc -s qdisc show` does.

* qdisc collector: nl-specific code moved, names fixed

- netlink-specific parts moved to github.com/ema/qdisc
- avoid using shortened names
- counters renamed into XXX_total

* Get rid of parseMessage error checking leftover

* Add github.com/ema/qdisc to vendored packages

* Update help texts and comments

* Add qdisc collector to README file

* qdisc collector end-to-end testing

* Update qdisc dependency to latest version

Update github.com/ema/qdisc dependency to revision 2c7e72d, which
includes unit testing.

* qdisc collector: rename "iface" label into "device"

collector/qdisc_linux.go

@@ -0,0 +1,116 @@
+// Copyright 2017 The Prometheus Authors
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+// +build !noqdisc
+
+package collector
+
+import (
+	"encoding/json"
+	"flag"
+	"io/ioutil"
+	"path/filepath"
+
+	"github.com/ema/qdisc"
+	"github.com/prometheus/client_golang/prometheus"
+)
+
+type qdiscStatCollector struct {
+	bytes      typedDesc
+	packets    typedDesc
+	drops      typedDesc
+	requeues   typedDesc
+	overlimits typedDesc
+}
+
+var (
+	collectorQdisc = flag.String("collector.qdisc", "", "test fixtures to use for qdisc collector end-to-end testing")
+)
+
+func init() {
+	Factories["qdisc"] = NewQdiscStatCollector
+}
+
+func NewQdiscStatCollector() (Collector, error) {
+	return &qdiscStatCollector{
+		bytes: typedDesc{prometheus.NewDesc(
+			prometheus.BuildFQName(Namespace, "qdisc", "bytes_total"),
+			"Number of bytes sent.",
+			[]string{"device", "kind"}, nil,
+		), prometheus.CounterValue},
+		packets: typedDesc{prometheus.NewDesc(
+			prometheus.BuildFQName(Namespace, "qdisc", "packets_total"),
+			"Number of packets sent.",
+			[]string{"device", "kind"}, nil,
+		), prometheus.CounterValue},
+		drops: typedDesc{prometheus.NewDesc(
+			prometheus.BuildFQName(Namespace, "qdisc", "drops_total"),
+			"Number of packets dropped.",
+			[]string{"device", "kind"}, nil,
+		), prometheus.CounterValue},
+		requeues: typedDesc{prometheus.NewDesc(
+			prometheus.BuildFQName(Namespace, "qdisc", "requeues_total"),
+			"Number of packets dequeued, not transmitted, and requeued.",
+			[]string{"device", "kind"}, nil,
+		), prometheus.CounterValue},
+		overlimits: typedDesc{prometheus.NewDesc(
+			prometheus.BuildFQName(Namespace, "qdisc", "overlimits_total"),
+			"Number of overlimit packets.",
+			[]string{"device", "kind"}, nil,
+		), prometheus.CounterValue},
+	}, nil
+}
+
+func testQdiscGet(fixtures string) ([]qdisc.QdiscInfo, error) {
+	var res []qdisc.QdiscInfo
+
+	b, err := ioutil.ReadFile(filepath.Join(fixtures, "results.json"))
+	if err != nil {
+		return res, err
+	}
+
+	err = json.Unmarshal(b, &res)
+	return res, err
+}
+
+func (c *qdiscStatCollector) Update(ch chan<- prometheus.Metric) error {
+	var msgs []qdisc.QdiscInfo
+	var err error
+
+	fixtures := *collectorQdisc
+
+	if fixtures == "" {
+		msgs, err = qdisc.Get()
+	} else {
+		msgs, err = testQdiscGet(fixtures)
+	}
+
+	if err != nil {
+		return err
+	}
+
+	for _, msg := range msgs {
+		// Only report root qdisc information.
+		if msg.Parent != 0 {
+			continue
+		}
+
+		ch <- c.bytes.mustNewConstMetric(float64(msg.Bytes), msg.IfaceName, msg.Kind)
+		ch <- c.packets.mustNewConstMetric(float64(msg.Packets), msg.IfaceName, msg.Kind)
+		ch <- c.drops.mustNewConstMetric(float64(msg.Drops), msg.IfaceName, msg.Kind)
+		ch <- c.requeues.mustNewConstMetric(float64(msg.Requeues), msg.IfaceName, msg.Kind)
+		ch <- c.overlimits.mustNewConstMetric(float64(msg.Overlimits), msg.IfaceName, msg.Kind)
+	}
+
+	return nil
+}