Fix rollover bug in mountstats collector (#1364)

* Update procfs vendor to pull in github.com/prometheus/procfs/pull/165
* Update mountstats collector to use new types.
* Rollover counter automatically to avoid float64 accuracy issues.
* Update e2e test.

Signed-off-by: Ben Kochie <superq@gmail.com>

vendor/github.com/prometheus/procfs/sysfs/clocksource.go

@@ -0,0 +1,76 @@
+// Copyright 2019 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 !windows
+
+package sysfs
+
+import (
+	"path/filepath"
+	"strings"
+
+	"github.com/prometheus/procfs/internal/util"
+)
+
+// ClockSource contains metrics related to the clock source
+type ClockSource struct {
+	Name      string
+	Available []string
+	Current   string
+}
+
+// ClockSources returns clocksource information including current and available clocksources
+// read from '/sys/devices/system/clocksource'
+func (fs FS) ClockSources() ([]ClockSource, error) {
+
+	clocksourcePaths, err := filepath.Glob(fs.sys.Path("devices/system/clocksource/clocksource[0-9]*"))
+	if err != nil {
+		return nil, err
+	}
+
+	clocksources := make([]ClockSource, len(clocksourcePaths))
+	for i, clocksourcePath := range clocksourcePaths {
+		clocksourceName := strings.TrimPrefix(filepath.Base(clocksourcePath), "clocksource")
+
+		clocksource, err := parseClocksource(clocksourcePath)
+		if err != nil {
+			return nil, err
+		}
+		clocksource.Name = clocksourceName
+		clocksources[i] = *clocksource
+	}
+
+	return clocksources, nil
+}
+
+func parseClocksource(clocksourcePath string) (*ClockSource, error) {
+
+	stringFiles := []string{
+		"available_clocksource",
+		"current_clocksource",
+	}
+	stringOut := make([]string, len(stringFiles))
+	var err error
+
+	for i, f := range stringFiles {
+		stringOut[i], err = util.SysReadFile(filepath.Join(clocksourcePath, f))
+		if err != nil {
+			return &ClockSource{}, err
+		}
+	}
+
+	return &ClockSource{
+		Available: strings.Fields(stringOut[0]),
+		Current:   stringOut[1],
+	}, nil
+}