Add missing Elasticsearch config deprecations

Several Elasticsearch config deprecations were overlooked for
monitoring-specific Elasticsearch settings.

Author: jportner

x-pack/legacy/plugins/monitoring/__tests__/deprecations.js

@@ -80,4 +80,54 @@ describe('monitoring plugin deprecations', function() {
       expect(log.called).to.be(true);
     });
   });
+
+  describe('elasticsearch.username', function() {
+    it('logs a warning if elasticsearch.username is set to "elastic"', () => {
+      const settings = { elasticsearch: { username: 'elastic' } };
+
+      const log = sinon.spy();
+      transformDeprecations(settings, log);
+      expect(log.called).to.be(true);
+    });
+
+    it('does not log a warning if elasticsearch.username is set to something besides "elastic"', () => {
+      const settings = { elasticsearch: { username: 'otheruser' } };
+
+      const log = sinon.spy();
+      transformDeprecations(settings, log);
+      expect(log.called).to.be(false);
+    });
+
+    it('does not log a warning if elasticsearch.username is unset', () => {
+      const settings = { elasticsearch: { username: undefined } };
+
+      const log = sinon.spy();
+      transformDeprecations(settings, log);
+      expect(log.called).to.be(false);
+    });
+
+    it('logs a warning if ssl.key is set and ssl.certificate is not', () => {
+      const settings = { elasticsearch: { ssl: { key: '' } } };
+
+      const log = sinon.spy();
+      transformDeprecations(settings, log);
+      expect(log.called).to.be(true);
+    });
+
+    it('logs a warning if ssl.certificate is set and ssl.key is not', () => {
+      const settings = { elasticsearch: { ssl: { certificate: '' } } };
+
+      const log = sinon.spy();
+      transformDeprecations(settings, log);
+      expect(log.called).to.be(true);
+    });
+
+    it('does not log a warning if both ssl.key and ssl.certificate are set', () => {
+      const settings = { elasticsearch: { ssl: { key: '', certificate: '' } } };
+
+      const log = sinon.spy();
+      transformDeprecations(settings, log);
+      expect(log.called).to.be(false);
+    });
+  });
 });

x-pack/legacy/plugins/monitoring/deprecations.js

@@ -26,6 +26,23 @@ export const deprecations = () => {
           `Config key "${CLUSTER_ALERTS_ADDRESS_CONFIG_KEY}" will be required for email notifications to work in 7.0."`
         );
       }
+
+      const fromPath = 'xpack.monitoring.elasticsearch';
+      const es = get(settings, 'elasticsearch');
+      if (es?.username === 'elastic') {
+        log(
+          `Setting [${fromPath}.username] to "elastic" is deprecated. You should use the "kibana" user instead.`
+        );
+      }
+      if (es?.ssl?.key !== undefined && es?.ssl?.certificate === undefined) {
+        log(
+          `Setting [${fromPath}.ssl.key] without [${fromPath}.ssl.certificate] is deprecated. This has no effect, you should use both settings to enable TLS client authentication to Elasticsearch.`
+        );
+      } else if (es?.ssl?.certificate !== undefined && es?.ssl?.key === undefined) {
+        log(
+          `Setting [${fromPath}.ssl.certificate] without [${fromPath}.ssl.key] is deprecated. This has no effect, you should use both settings to enable TLS client authentication to Elasticsearch.`
+        );
+      }
     },
   ];
 };