Add retries to getServer() (prometheus-community#316)

Some backstory ============== I was attempting to use postgres_exporter with the official Docker container (https://hub.docker.com/_/postgres) In a Kubernetes StatefulSet, with a side-car configuration, but found that I wasn't able to connect even with sharing the Postgres Unix listening socket, between both containers. After copying the container over to an Alpine base I quickly found out that the postgres_exporter was actually starting before the main Postres container had dropped the unix socket onto the file system, a quick work around is to write a bash for loop checking for the existence of a unix socket, however this would require maintaining a container, besides other users may find retries useful on startup. Implementation ============== All changes are made to the getServer function and variables are local, I was unsure if it was worth adding command line switches but this would allow for a more sophisticated backOff loop in the future. Hope this help, and let me know if you would like me to changes anything.

Author: dmolik

cmd/postgres_exporter/postgres_exporter.go

@@ -863,17 +863,29 @@ func (s *Servers) GetServer(dsn string) (*Server, error) {
 s.m.Lock()
 defer s.m.Unlock()
 var err error
-server, ok := s.servers[dsn]
-if !ok {
-server, err = NewServer(dsn, s.opts...)
-if err != nil {
+var ok bool
+errCount := 0 // start at zero because we increment before doing work
+retries := 3
+var server *Server
+for {
+if errCount++; errCount > retries {
 return nil, err
 }
-s.servers[dsn] = server
-}
-if err = server.Ping(); err != nil {
-delete(s.servers, dsn)
-return nil, err
+server, ok = s.servers[dsn]
+if !ok {
+server, err = NewServer(dsn, s.opts...)
+if err != nil {
+time.Sleep(time.Duration(errCount) * time.Second)
+continue
+}
+s.servers[dsn] = server
+}
+if err = server.Ping(); err != nil {
+delete(s.servers, dsn)
+time.Sleep(time.Duration(errCount) * time.Second)
+continue
+}
+break
 }
 return server, nil
 }