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

service/src/main/java/org/whispersystems/textsecuregcm/util/CertificateExpirationGauge.java

@@ -0,0 +1,32 @@
+package org.whispersystems.textsecuregcm.util;
+
+import com.codahale.metrics.CachedGauge;
+import org.bouncycastle.openssl.PEMReader;
+
+import java.io.ByteArrayInputStream;
+import java.io.IOException;
+import java.io.InputStreamReader;
+import java.security.cert.CertificateException;
+import java.security.cert.X509Certificate;
+import java.time.Duration;
+import java.time.Instant;
+import java.util.concurrent.TimeUnit;
+
+/**
+ * Measures and reports the number of days until a certificate expires.
+ */
+public class CertificateExpirationGauge extends CachedGauge<Long> {
+
+    private final Instant certificateExpiration;
+
+    public CertificateExpirationGauge(final X509Certificate certificate) {
+        super(1, TimeUnit.HOURS);
+
+        certificateExpiration = certificate.getNotAfter().toInstant();
+    }
+
+    @Override
+    protected Long loadValue() {
+        return Duration.between(Instant.now(), certificateExpiration).toDays();
+    }
+}