Support checking configuration and exiting

If desired, go through all the motions of setting up the exporter, but
then exit. This is not very elegant, as we actually open the ports,
which may not be what you want to do in a pipeline, but it is the
quickest way to implement this.

Fixes #263.

Signed-off-by: Matthias Rampke <matthias@prometheus.io>

README.md

@@ -81,42 +81,56 @@ NOTE: Version 0.7.0 switched to the [kingpin](https://github.com/alecthomas/king
 * flag processing stops at the first `--`
 
     ```
-    $ go build
-    $ ./statsd_exporter --help
     usage: statsd_exporter [<flags>]
 
     Flags:
-      -h, --help                    Show context-sensitive help (also try --help-long and --help-man).
+      -h, --help                    Show context-sensitive help (also try
+                                    --help-long and --help-man).
           --web.listen-address=":9102"
-                                    The address on which to expose the web interface and generated Prometheus     metrics.
+                                    The address on which to expose the web interface
+                                    and generated Prometheus metrics.
           --web.telemetry-path="/metrics"
                                     Path under which to expose metrics.
           --statsd.listen-udp=":9125"
-                                    The UDP address on which to receive statsd metric lines. "" disables it.
+                                    The UDP address on which to receive statsd
+                                    metric lines. "" disables it.
           --statsd.listen-tcp=":9125"
-                                    The TCP address on which to receive statsd metric lines. "" disables it.
+                                    The TCP address on which to receive statsd
+                                    metric lines. "" disables it.
           --statsd.listen-unixgram=""
-                                    The Unixgram socket path to receive statsd metric lines in datagram. ""     disables it.
+                                    The Unixgram socket path to receive statsd
+                                    metric lines in datagram. "" disables it.
           --statsd.unixsocket-mode="755"
                                     The permission mode of the unix socket.
           --statsd.mapping-config=STATSD.MAPPING-CONFIG
                                     Metric mapping configuration file name.
           --statsd.read-buffer=STATSD.READ-BUFFER
-                                    Size (in bytes) of the operating system's transmit read buffer associated     with the UDP or Unixgram connection. Please make sure the kernel     parameters net.core.rmem_max is set to
+                                    Size (in bytes) of the operating system's
-                                    a value greater than the value specified.
+                                    transmit read buffer associated with the UDP or
-          --statsd.cache-size=1000  Maximum size of your metric mapping cache. Relies on least recently used     replacement policy if max size is reached.
+                                    Unixgram connection. Please make sure the kernel
+                                    parameters net.core.rmem_max is set to a value
+                                    greater than the value specified.
+          --statsd.cache-size=1000  Maximum size of your metric mapping cache.
+                                    Relies on least recently used replacement policy
+                                    if max size is reached.
+          --statsd.cache-type=lru   Metric mapping cache type. Valid options are
+                                    "lru" and "random"
           --statsd.event-queue-size=10000
                                     Size of internal queue for processing events
           --statsd.event-flush-threshold=1000
-                                    Number of events to hold in queue before flushing
+                                    Number of events to hold in queue before
+                                    flushing
           --statsd.event-flush-interval=200ms
-                                    Number of events to hold in queue before flushing
+                                    Number of events to hold in queue before
-          --debug.dump-fsm=""       The path to dump internal FSM generated for glob matching as Dot file.
+                                    flushing
-          --log.level="info"        Only log messages with the given severity or above. Valid levels: [debug,     info, warn, error, fatal]
+          --debug.dump-fsm=""       The path to dump internal FSM generated for glob
-          --log.format="logger:stderr"
+                                    matching as Dot file.
-                                    Set the log target and format. Example: "logger:syslog?appname=bob&    local=7" or "logger:stdout?json=true"
+          --check-config            Check configuration and exit.
+          --log.level=info          Only log messages with the given severity or
+                                    above. One of: [debug, info, warn, error]
+          --log.format=logfmt       Output format of log messages. One of: [logfmt,
+                                    json]
           --version                 Show application version.
-
     ```
 
 ## Tests
@@ -207,8 +221,8 @@ mappings:
     provider: "$1"
 ```
 
-Be aware about yaml escape rules as a mapping like the following one will not work. 
+Be aware about yaml escape rules as a mapping like the following one will not work.
-```yaml 
+```yaml
 mappings:
 - match: "test\.(\w+)\.(\w+)\.counter"
   match_type: regex

main.go

@@ -267,6 +267,7 @@ func main() {
 		eventFlushThreshold  = kingpin.Flag("statsd.event-flush-threshold", "Number of events to hold in queue before flushing").Default("1000").Int()
 		eventFlushInterval   = kingpin.Flag("statsd.event-flush-interval", "Number of events to hold in queue before flushing").Default("200ms").Duration()
 		dumpFSMPath          = kingpin.Flag("debug.dump-fsm", "The path to dump internal FSM generated for glob matching as Dot file.").Default("").String()
+		checkConfig          = kingpin.Flag("check-config", "Check configuration and exit.").Default("false").Bool()
 	)
 
 	promlogConfig := &promlog.Config{}
@@ -440,13 +441,17 @@ func main() {
 		mapper.InitCache(*cacheSize, cacheOption)
 	}
 
-	go configReloader(*mappingConfig, mapper, *cacheSize, logger, cacheOption)
-
 	exporter := exporter.NewExporter(mapper, logger, eventsActions, eventsUnmapped, errorEventStats, eventStats, conflictingEventStats, metricsCount)
 
+	if *checkConfig {
+		level.Info(logger).Log("msg", "Configuration check successful, exiting")
+		return
+	}
+
 	signals := make(chan os.Signal, 1)
 	signal.Notify(signals, os.Interrupt, syscall.SIGTERM)
 
+	go configReloader(*mappingConfig, mapper, *cacheSize, logger, cacheOption)
 	go exporter.Listen(events)
 
 	<-signals