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/fixtures/e2e-output.txt

@@ -2114,6 +2114,26 @@ node_procs_blocked 0
 # HELP node_procs_running Number of processes in runnable state.
 # TYPE node_procs_running gauge
 node_procs_running 2
+# HELP node_qdisc_bytes_total Number of bytes sent.
+# TYPE node_qdisc_bytes_total counter
+node_qdisc_bytes_total{device="eth0",kind="pfifo_fast"} 83
+node_qdisc_bytes_total{device="wlan0",kind="fq"} 42
+# HELP node_qdisc_drops_total Number of packets dropped.
+# TYPE node_qdisc_drops_total counter
+node_qdisc_drops_total{device="eth0",kind="pfifo_fast"} 0
+node_qdisc_drops_total{device="wlan0",kind="fq"} 1
+# HELP node_qdisc_overlimits_total Number of overlimit packets.
+# TYPE node_qdisc_overlimits_total counter
+node_qdisc_overlimits_total{device="eth0",kind="pfifo_fast"} 0
+node_qdisc_overlimits_total{device="wlan0",kind="fq"} 0
+# HELP node_qdisc_packets_total Number of packets sent.
+# TYPE node_qdisc_packets_total counter
+node_qdisc_packets_total{device="eth0",kind="pfifo_fast"} 83
+node_qdisc_packets_total{device="wlan0",kind="fq"} 42
+# HELP node_qdisc_requeues_total Number of packets dequeued, not transmitted, and requeued.
+# TYPE node_qdisc_requeues_total counter
+node_qdisc_requeues_total{device="eth0",kind="pfifo_fast"} 2
+node_qdisc_requeues_total{device="wlan0",kind="fq"} 1
 # HELP node_scrape_collector_duration_seconds node_exporter: Duration of a collector scrape.
 # TYPE node_scrape_collector_duration_seconds gauge
 # HELP node_scrape_collector_success node_exporter: Whether a collector succeeded.
@@ -2139,6 +2159,7 @@ node_scrape_collector_success{collector="mountstats"} 1
 node_scrape_collector_success{collector="netdev"} 1
 node_scrape_collector_success{collector="netstat"} 1
 node_scrape_collector_success{collector="nfs"} 1
+node_scrape_collector_success{collector="qdisc"} 1
 node_scrape_collector_success{collector="sockstat"} 1
 node_scrape_collector_success{collector="stat"} 1
 node_scrape_collector_success{collector="textfile"} 1

collector/fixtures/qdisc/results.json

@@ -0,0 +1,17 @@
+[
+    {
+        "IfaceName": "wlan0",
+        "Bytes": 42,
+        "Packets": 42,
+        "Requeues": 1,
+        "Kind": "fq",
+        "Drops": 1
+    },
+    {
+        "IfaceName": "eth0",
+        "Bytes": 83,
+        "Packets": 83,
+        "Requeues": 2,
+        "Kind": "pfifo_fast"
+    }
+]