Add env var to override introspection bind address

2b08772 (inadvertently?) changed the
bind address from `:61678` to `localhost:61678` which is backwards
incompatible and for those actually scraping the metrics via prometheus
makes the metrics endpoint entirely unusable (since prometheus isn't
local to each node in the cluster).

This path simply adds an env var to override the bind address.

ipamd/introspect.go

@@ -27,8 +27,11 @@ import (
 )
 
 const (
-	// introspectionAddress is listening on localhost 61679 for ipamd introspection
-	introspectionAddress = "127.0.0.1:61679"
+	// defaultIntrospectionAddress is listening on localhost 61679 for ipamd introspection
+	defaultIntrospectionBindAddress = "127.0.0.1:61679"
+
+	// Environment variable to define the bind address for the introspection endpoint
+	introspectionBindAddress = "INTROSPECTION_BIND_ADDRESS"
 
 	// Environment variable to disable the introspection endpoints
 	envDisableIntrospection = "DISABLE_INTROSPECTION"
@@ -55,7 +58,6 @@ func (c *IPAMContext) ServeIntrospection() {
 		return
 	}
 
-	log.Info("Serving introspection endpoints on ", introspectionAddress)
 	server := c.setupIntrospectionServer()
 	for {
 		once := sync.Once{}
@@ -102,8 +104,15 @@ func (c *IPAMContext) setupIntrospectionServer() *http.Server {
 	loggingServeMux := http.NewServeMux()
 	loggingServeMux.Handle("/", LoggingHandler{serveMux})
 
+	addr, ok := os.LookupEnv(introspectionBindAddress)
+	if !ok {
+		addr = defaultIntrospectionBindAddress
+	}
+
+	log.Info("Serving introspection endpoints on ", addr)
+
 	server := &http.Server{
-		Addr:         introspectionAddress,
+		Addr:         addr,
 		Handler:      loggingServeMux,
 		ReadTimeout:  5 * time.Second,
 		WriteTimeout: 5 * time.Second,