DropWizard metric support

Gerrit server supports defining and recording metrics.  Metric
reporters for monitoring can be implemented as plugins.  A basic
Graphite reporter is available here:

  https://gerrit-review.googlesource.com/#/c/72202/

Some example metrics are included in this change:

  change/query/query_latency
  (Query latency)

  sshd/sessions/connected
  (SSH sessions connected)

  sshd/sessions/created/count
  (SSH connections created)

  git/upload-pack
  (Upload packs requests)

Partially-by: Gustaf Lundh <gustaflh@axis.com>
Change-Id: I46a07aace57efe236ee724ec8d34c581e2c37965
diff --git a/gerrit-sshd/BUCK b/gerrit-sshd/BUCK
index dcff98e..279b024 100644
--- a/gerrit-sshd/BUCK
+++ b/gerrit-sshd/BUCK
@@ -21,6 +21,7 @@
     '//lib/auto:auto-value',
     '//lib/commons:codec',
     '//lib/commons:collections',
+    '//lib/dropwizard:dropwizard-core',
     '//lib/guice:guice',
     '//lib/guice:guice-assistedinject',
     '//lib/guice:guice-servlet',  # SSH should not depend on servlet