feat: support for textfile metrics (#13)

Add support for collecting arbitrary metrics from a textfile as well as
metrics collected from fail2ban. This allows other data to be exported
along with the fail2ban metrics (e.g. instance metadata).
Update the docker image to allow mounting a folder with a collection of
metric files to be exported. Only files ending in `.prom` with be read.
Update project README with the new functionality.

src/cfg/cfg.go

@@ -12,10 +12,12 @@ const (
 )
 
 type AppSettings struct {
-	VersionMode        bool
-	MetricsPort        int
-	Fail2BanDbPath     string
-	Fail2BanSocketPath string
+	VersionMode          bool
+	MetricsPort          int
+	Fail2BanDbPath       string
+	Fail2BanSocketPath   string
+	FileCollectorPath    string
+	FileCollectorEnabled bool
 }
 
 func Parse() *AppSettings {
@@ -24,6 +26,8 @@ func Parse() *AppSettings {
 	flag.IntVar(&appSettings.MetricsPort, "port", 9191, "port to use for the metrics server")
 	flag.StringVar(&appSettings.Fail2BanDbPath, "db", "", "path to the fail2ban sqlite database (deprecated)")
 	flag.StringVar(&appSettings.Fail2BanSocketPath, "socket", "", "path to the fail2ban server socket")
+	flag.BoolVar(&appSettings.FileCollectorEnabled, "collector.textfile", false, "enable the textfile collector")
+	flag.StringVar(&appSettings.FileCollectorPath, "collector.textfile.directory", "", "directory to read text files with metrics from")
 
 	flag.Parse()
 	appSettings.validateFlags()
@@ -42,6 +46,10 @@ func (settings *AppSettings) validateFlags() {
 				minServerPort, maxServerPort, settings.MetricsPort)
 			flagsValid = false
 		}
+		if settings.FileCollectorEnabled && settings.FileCollectorPath == "" {
+			fmt.Printf("file collector directory path must not be empty if collector enabled\n")
+			flagsValid = false
+		}
 	}
 	if !flagsValid {
 		flag.Usage()

src/exporter.go

@@ -3,6 +3,7 @@ package main
 import (
 	"fail2ban-prometheus-exporter/cfg"
 	"fail2ban-prometheus-exporter/export"
+	"fail2ban-prometheus-exporter/textfile"
 	"fmt"
 	"log"
 	"net/http"
@@ -43,6 +44,11 @@ func rootHtmlHandler(w http.ResponseWriter, r *http.Request) {
 	}
 }
 
+func metricHandler(w http.ResponseWriter, r *http.Request, collector *textfile.Collector) {
+	promhttp.Handler().ServeHTTP(w, r)
+	collector.WriteTextFileMetrics(w, r)
+}
+
 func main() {
 	appSettings := cfg.Parse()
 	if appSettings.VersionMode {
@@ -55,8 +61,13 @@ func main() {
 		exporter := export.NewExporter(appSettings, version)
 		prometheus.MustRegister(exporter)
 
+		textFileCollector := textfile.NewCollector(appSettings)
+		prometheus.MustRegister(textFileCollector)
+
 		http.HandleFunc("/", rootHtmlHandler)
-		http.Handle(metricsPath, promhttp.Handler())
+		http.HandleFunc(metricsPath, func(w http.ResponseWriter, r *http.Request) {
+			metricHandler(w, r, textFileCollector)
+		})
 		log.Printf("metrics available at '%s'", metricsPath)
 
 		svrErr := make(chan error)

src/textfile/collector.go

@@ -0,0 +1,48 @@
+package textfile
+
+import (
+	"fail2ban-prometheus-exporter/cfg"
+	"github.com/prometheus/client_golang/prometheus"
+	"log"
+)
+
+type Collector struct {
+	enabled    bool
+	folderPath string
+	fileMap    map[string]*fileData
+}
+
+type fileData struct {
+	readErrors   int
+	fileName     string
+	fileContents []byte
+}
+
+func NewCollector(appSettings *cfg.AppSettings) *Collector {
+	collector := &Collector{
+		enabled:    appSettings.FileCollectorEnabled,
+		folderPath: appSettings.FileCollectorPath,
+		fileMap:    make(map[string]*fileData),
+	}
+	if collector.enabled {
+		log.Printf("collector.textfile directory: %s", collector.folderPath)
+	}
+	return collector
+}
+
+func (c *Collector) Describe(ch chan<- *prometheus.Desc) {
+	if c.enabled {
+		ch <- metricReadError
+	}
+}
+
+func (c *Collector) Collect(ch chan<- prometheus.Metric) {
+	if c.enabled {
+		c.collectFileContents()
+		c.collectFileErrors(ch)
+	}
+}
+
+func (c *Collector) appendErrorForPath(path string) {
+	c.fileMap[path].readErrors++
+}

src/textfile/file.go

@@ -0,0 +1,55 @@
+package textfile
+
+import (
+	"github.com/prometheus/client_golang/prometheus"
+	"io/ioutil"
+	"log"
+	"path/filepath"
+	"strings"
+)
+
+const namespace = "textfile"
+
+var (
+	metricReadError = prometheus.NewDesc(
+		prometheus.BuildFQName(namespace, "", "error"),
+		"Checks for errors while reading text files",
+		[]string{"path"}, nil,
+	)
+)
+
+func (c *Collector) collectFileContents() {
+	files, err := ioutil.ReadDir(c.folderPath)
+	if err != nil {
+		log.Printf("error reading directory '%s': %v", c.folderPath, err)
+		return
+	}
+
+	for _, file := range files {
+		fileName := file.Name()
+		if !strings.HasSuffix(strings.ToLower(fileName), ".prom") {
+			continue
+		}
+		c.fileMap[fileName] = &fileData{
+			readErrors: 0,
+			fileName:   fileName,
+		}
+
+		fullPath := filepath.Join(c.folderPath, fileName)
+		content, err := ioutil.ReadFile(fullPath)
+		if err != nil {
+			c.appendErrorForPath(fileName)
+			log.Printf("error reading contents of file '%s': %v", fileName, err)
+		}
+
+		c.fileMap[fileName].fileContents = content
+	}
+}
+
+func (c *Collector) collectFileErrors(ch chan<- prometheus.Metric) {
+	for _, f := range c.fileMap {
+		ch <- prometheus.MustNewConstMetric(
+			metricReadError, prometheus.GaugeValue, float64(f.readErrors), f.fileName,
+		)
+	}
+}

src/textfile/writer.go

@@ -0,0 +1,20 @@
+package textfile
+
+import (
+	"log"
+	"net/http"
+)
+
+func (c *Collector) WriteTextFileMetrics(w http.ResponseWriter, r *http.Request) {
+	if !c.enabled {
+		return
+	}
+
+	for _, f := range c.fileMap {
+		_, err := w.Write(f.fileContents)
+		if err != nil {
+			c.appendErrorForPath(f.fileName)
+			log.Printf("error writing file contents to response writer '%s': %v", f.fileName, err)
+		}
+	}
+}