Record metrics for WHOIS commands

Note that this does not write out metrics for invocations of the
nomulus tool.

This requires a slight refactoring of the existing WhoisResponse
interface so as to also support returning the number of results found
by the WHOIS query.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=149461208

java/google/registry/whois/Whois.java

@@ -46,9 +46,10 @@ public final class Whois {
           .create(now)
           .readCommand(new StringReader(query))
           .executeQuery(now)
-          .getPlainTextOutput(preferUnicode, disclaimer);
+          .getResponse(preferUnicode, disclaimer)
+          .plainTextOutput();
     } catch (WhoisException e) {
-      return e.getPlainTextOutput(preferUnicode, disclaimer);
+      return e.getResponse(preferUnicode, disclaimer).plainTextOutput();
     } catch (IOException e) {
       throw new RuntimeException(e);
     }