Fix BlockedThreadsCheck failing with un-blocked threads

The introduction of BlockedThreadsCheck introduced a P0
regression because of the accumulatio of the counter of
blocked threads caused any Gerrit instance to eventually
fail once the counter of blocked threads reached the 50%
of the total number.

Remove the @Singleton annotation misplaced on the check
and make sure that similar issues aren't passed unnoticed
by using the actual Guice injection for creating the
BlockedThreadsCheck in tests rather than calling the
constructor directly.

Bug: Issue 14736
Change-Id: I430a7a576f8c41594c59d71cced6d43d7acf985a

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

diff --git a/src/main/java/com/googlesource/gerrit/plugins/healthcheck/check/BlockedThreadsCheck.java b/src/main/java/com/googlesource/gerrit/plugins/healthcheck/check/BlockedThreadsCheck.java
index f2cbb70..4e21771 100644
--- a/src/main/java/com/googlesource/gerrit/plugins/healthcheck/check/BlockedThreadsCheck.java
+++ b/src/main/java/com/googlesource/gerrit/plugins/healthcheck/check/BlockedThreadsCheck.java
@@ -23,7 +23,6 @@
 import com.google.inject.Inject;
 import com.google.inject.Module;
 import com.google.inject.Provider;
-import com.google.inject.Singleton;
 import com.googlesource.gerrit.plugins.healthcheck.HealthCheckConfig;
 import com.googlesource.gerrit.plugins.healthcheck.HealthCheckMetrics;
 import java.lang.management.ManagementFactory;
@@ -35,7 +34,6 @@
 import java.util.function.Supplier;
 import java.util.stream.Stream;
 
-@Singleton
 public class BlockedThreadsCheck extends AbstractHealthCheck {
   public static Module SUB_CHECKS =
       new FactoryModule() {

src/test/java/com/googlesource/gerrit/plugins/healthcheck/BlockedThreadsCheckTest.java

diff --git a/src/test/java/com/googlesource/gerrit/plugins/healthcheck/BlockedThreadsCheckTest.java b/src/test/java/com/googlesource/gerrit/plugins/healthcheck/BlockedThreadsCheckTest.java
index 641a060..c52d646 100644
--- a/src/test/java/com/googlesource/gerrit/plugins/healthcheck/BlockedThreadsCheckTest.java
+++ b/src/test/java/com/googlesource/gerrit/plugins/healthcheck/BlockedThreadsCheckTest.java
@@ -93,6 +93,27 @@
   }
 
   @Test
+  public void shouldFailThenPassCheck() {
+    String prefix = "some-prefix";
+
+    List<ThreadInfo> failingThreadInfo = new ArrayList<>();
+    failingThreadInfo.addAll(nCopies(1, mockInfo(Thread.State.RUNNABLE, prefix)));
+    failingThreadInfo.addAll(nCopies(3, mockInfo(Thread.State.BLOCKED, prefix)));
+
+    List<ThreadInfo> successThreadInfo = new ArrayList<>();
+    successThreadInfo.addAll(nCopies(1, mockInfo(Thread.State.RUNNABLE, prefix)));
+    successThreadInfo.addAll(nCopies(1, mockInfo(Thread.State.BLOCKED, prefix)));
+
+    when(beanMock.getThreadInfo(null, 0))
+        .thenReturn(
+            failingThreadInfo.toArray(new ThreadInfo[0]),
+            successThreadInfo.toArray(new ThreadInfo[0]));
+
+    assertThat(createCheck().run().result).isEqualTo(Result.FAILED);
+    assertThat(createCheck().run().result).isEqualTo(Result.PASSED);
+  }
+
+  @Test
   public void shouldPassCheckWhenBlockedThreadsAreLessThenThreshold() {
     int running = 3;
     int blocked = 1;