twitter-server: Add filtering to /admin/registry.json

Problem

There are times you are interested in a very specific subsection of
the registry and you don't feel like using `jq` or parsing the output.

Solution

Introduce an optional HTTP request parameter `filter` allows for
simple filtering of the returned data.  The value takes the form
"path/to/data" and returns registry entries whose prefix matches the
parameter. A glob, `*`, can be used to match any component of the
path. For example, using "filter=registry/server/http" will output all
Finagle Http servers.  Using "filter=registry/client/\*/user_service"
would output all Finagle clients with a label of "user_service"
regardless of protocol.

RB_ID=842784

util-registry/src/main/scala/com/twitter/util/registry/Formatter.scala

@@ -6,6 +6,11 @@ import scala.util.control.NoStackTrace
 object Formatter {
   private[this] val log = Logger.getLogger(getClass.getName)
 
+  /**
+   * The top-level key in the `Map` returned by [[asMap]].
+   */
+  val RegistryKey = "registry"
+
   private[registry] val Eponymous = "__eponymous"
 
   /**
@@ -60,7 +65,7 @@ object Formatter {
           log.severe(s"incorrectly found an empty seq on $returnString")
       }
     }
-    Map("registry" -> map)
+    Map(RegistryKey -> map)
   }
 
   private[registry] val Collision = new Exception() with NoStackTrace