[KYUUBI #6891] Fix get existing gauge issue

### Why are the changes needed?

For the `com.codahale.metrics.MetricRegistry::gauge`.
It `getOrAdd` the gauge with name.
```
    public <T extends Gauge> T gauge(String name) {
        return (Gauge)this.getOrAdd(name, MetricRegistry.MetricBuilder.GAUGES);
    }
```

So we have to get all the gauges to check whether the gauge exists.

### How was this patch tested?

UT.
### Was this patch authored or co-authored using generative AI tooling?

No.

Closes #6891 from turboFei/gauge_exists.

Closes #6891

18be2a5 [Wang, Fei] o(1)
039e7b5 [Wang, Fei] check existing gauge
32dce6f [Wang, Fei] check gauge exists

Authored-by: Wang, Fei <[email protected]>
Signed-off-by: Wang, Fei <[email protected]>

Author: turboFei

kyuubi-metrics/src/main/scala/org/apache/kyuubi/metrics/MetricsSystem.scala

@@ -58,8 +58,8 @@ class MetricsSystem extends CompositeService("MetricsSystem") {
     meter.mark(value)
   }
 
-  def getGauge[T](name: String): Option[Gauge[T]] = {
-    Option(registry.gauge(name))
+  def getGauge(name: String): Option[Gauge[_]] = {
+    Option(registry.getGauges().get(name))
   }
 
   def registerGauge[T](name: String, value: => T, default: T): Unit = {

kyuubi-metrics/src/test/scala/org/apache/kyuubi/metrics/MetricsSystemSuite.scala

@@ -94,4 +94,20 @@ class MetricsSystemSuite extends KyuubiFunSuite {
     checkJsonFileMetrics(reportFile, "20181117")
     metricsSystem.stop()
   }
+
+  test("metrics - get gauge") {
+    val conf = KyuubiConf().set(MetricsConf.METRICS_ENABLED, true)
+    val metricsSystem = new MetricsSystem()
+    metricsSystem.initialize(conf)
+    metricsSystem.start()
+
+    assert(metricsSystem.getGauge(MetricsConstants.THRIFT_SSL_CERT_EXPIRATION).isEmpty)
+    metricsSystem.registerGauge(
+      MetricsConstants.THRIFT_SSL_CERT_EXPIRATION,
+      1000,
+      0)
+    assert(metricsSystem.getGauge(MetricsConstants.THRIFT_SSL_CERT_EXPIRATION).get.getValue == 1000)
+
+    metricsSystem.stop()
+  }
 }