Vtgateproxy rework discovery and add metrics (#296)

* refactor a bit more to consolidate the command line flags

* rework again to have a single parser and move around all the logic

This way we only have a single entity watching the file and dispatching out to
the resolvers when things change rather than a bunch of tickers watching the
file. Also cleaned up the code a bunch.

* redo the shuffle to be a single pass

* actually use the builder rand not the global one

* add some metrics

* only stat once per second

* split out counter for unknown affinity

go/cmd/vtgateproxy/vtgateproxy.go

@@ -17,17 +17,13 @@ limitations under the License.
 package main
 
 import (
-	"math/rand"
-	"time"
-
 	"vitess.io/vitess/go/exit"
 	"vitess.io/vitess/go/stats/prometheusbackend"
 	"vitess.io/vitess/go/vt/servenv"
 	"vitess.io/vitess/go/vt/vtgateproxy"
 )
 
 func init() {
-	rand.Seed(time.Now().UnixNano())
 	servenv.RegisterDefaultFlags()
 }
 
@@ -40,8 +36,6 @@ func main() {
 	prometheusbackend.Init("vtgateproxy")
 
 	servenv.OnRun(func() {
-		// Flags are parsed now. Parse the template using the actual flag value and overwrite the current template.
-		vtgateproxy.RegisterJsonDiscovery()
 		vtgateproxy.Init()
 	})