Add missing Singleton to ActiveWokersCheck and DeadlockCheck

The ActiveWorkersCheck was triggering the creation
of a new check for every incoming REST-API call, causing
potential deadlocks because of the creation of the
DropWizard's MetricMaker.

As a consequence, the Gerrit instance could have ran
out of incoming HTTP threads.

See as example stack trace:

HTTP GET /config/server/healthcheck~status
   java.lang.Thread.State: BLOCKED (on object monitor)
	at com.google.gerrit.metrics.dropwizard.DropWizardMetricMaker.newCounter(DropWizardMetricMaker.java:142)
	- waiting to lock <0x00007f828e8b80b0> (a com.google.gerrit.metrics.dropwizard.DropWizardMetricMaker)
	at com.google.gerrit.server.plugins.PluginMetricMaker.newCounter(PluginMetricMaker.java:55)
	at com.googlesource.gerrit.plugins.healthcheck.HealthCheckMetrics.getFailureCounterMetric(HealthCheckMetrics.java:33)
	at com.googlesource.gerrit.plugins.healthcheck.check.AbstractHealthCheck.<init>(AbstractHealthCheck.java:54)
	at com.googlesource.gerrit.plugins.healthcheck.check.ActiveWorkersCheck.<init>(ActiveWorkersCheck.java:46)

Change-Id: I947b46b58e1d44840b3a44f740627e55dc3194aa

src/main/java/com/googlesource/gerrit/plugins/healthcheck/check/ActiveWorkersCheck.java

diff --git a/src/main/java/com/googlesource/gerrit/plugins/healthcheck/check/ActiveWorkersCheck.java b/src/main/java/com/googlesource/gerrit/plugins/healthcheck/check/ActiveWorkersCheck.java
index 8684ef8..5fe83e4 100644
--- a/src/main/java/com/googlesource/gerrit/plugins/healthcheck/check/ActiveWorkersCheck.java
+++ b/src/main/java/com/googlesource/gerrit/plugins/healthcheck/check/ActiveWorkersCheck.java
@@ -22,10 +22,12 @@
 import com.google.gerrit.server.config.GerritServerConfig;
 import com.google.gerrit.server.config.ThreadSettingsConfig;
 import com.google.inject.Inject;
+import com.google.inject.Singleton;
 import com.googlesource.gerrit.plugins.healthcheck.HealthCheckConfig;
 import java.util.Optional;
 import org.eclipse.jgit.lib.Config;
 
+@Singleton
 public class ActiveWorkersCheck extends AbstractHealthCheck {
 
   public static final String ACTIVE_WORKERS_METRIC_NAME =

src/main/java/com/googlesource/gerrit/plugins/healthcheck/check/DeadlockCheck.java

diff --git a/src/main/java/com/googlesource/gerrit/plugins/healthcheck/check/DeadlockCheck.java b/src/main/java/com/googlesource/gerrit/plugins/healthcheck/check/DeadlockCheck.java
index 3d4b672..9caca0e 100644
--- a/src/main/java/com/googlesource/gerrit/plugins/healthcheck/check/DeadlockCheck.java
+++ b/src/main/java/com/googlesource/gerrit/plugins/healthcheck/check/DeadlockCheck.java
@@ -20,9 +20,11 @@
 import com.google.common.util.concurrent.ListeningExecutorService;
 import com.google.gerrit.metrics.MetricMaker;
 import com.google.inject.Inject;
+import com.google.inject.Singleton;
 import com.googlesource.gerrit.plugins.healthcheck.HealthCheckConfig;
 import java.util.Optional;
 
+@Singleton
 public class DeadlockCheck extends AbstractHealthCheck {
 
   public static final String DEADLOCKED_THREADS_METRIC_NAME =