Report the number of days until the CDS CA cert expires as a metric so we can set an alarm.

service/src/test/java/org/whispersystems/textsecuregcm/util/CertificateExpirationGaugeTest.java

@@ -0,0 +1,31 @@
+package org.whispersystems.textsecuregcm.util;
+
+import org.junit.Test;
+
+import java.security.cert.X509Certificate;
+import java.time.Duration;
+import java.time.Instant;
+import java.util.Date;
+
+import static org.junit.Assert.*;
+import static org.mockito.Mockito.mock;
+import static org.mockito.Mockito.when;
+
+public class CertificateExpirationGaugeTest {
+
+    @Test
+    public void loadValue() {
+        final X509Certificate            certificate         = mock(X509Certificate.class);
+
+        final long                       daysUntilExpiration = 17;
+
+        final Instant                    now                 = Instant.now();
+        final Instant                    later               = now.plus(Duration.ofDays(daysUntilExpiration)).plus(Duration.ofMinutes(1));
+
+        when(certificate.getNotAfter()).thenReturn(new Date(later.toEpochMilli()));
+
+        final CertificateExpirationGauge gauge               = new CertificateExpirationGauge(certificate);
+
+        assertEquals(daysUntilExpiration, (long) gauge.loadValue());
+    }
+}