[SPARK-30060][CORE] Rename metrics enable/disable configs

### What changes were proposed in this pull request?
This proposes to introduce a naming convention for Spark metrics configuration parameters used to enable/disable metrics source reporting using the Dropwizard metrics library:   `spark.metrics.sourceNameCamelCase.enabled` and update 2 parameters to use this naming convention.

### Why are the changes needed?
Currently Spark has a few parameters to enable/disable metrics reporting. Their naming pattern is not uniform and this can create confusion.  Currently we have:
`spark.metrics.static.sources.enabled`
`spark.app.status.metrics.enabled`
`spark.sql.streaming.metricsEnabled`

### Does this PR introduce any user-facing change?
Update parameters for enabling/disabling metrics reporting new in Spark 3.0: `spark.metrics.static.sources.enabled` -> `spark.metrics.staticSources.enabled`, `spark.app.status.metrics.enabled`  -> `spark.metrics.appStatusSource.enabled`.
Note: `spark.sql.streaming.metricsEnabled` is left unchanged as it is already in use in Spark 2.x.

### How was this patch tested?
Manually tested

Closes #26692 from LucaCanali/uniformNamingMetricsEnableParameters.

Authored-by: Luca Canali <luca.canali@cern.ch>
Signed-off-by: Dongjoon Hyun <dhyun@apple.com>

core/src/main/scala/org/apache/spark/internal/config/Status.scala

@@ -55,8 +55,8 @@ private[spark] object Status {
     .intConf
     .createWithDefault(Int.MaxValue)
 
-  val APP_STATUS_METRICS_ENABLED =
-    ConfigBuilder("spark.app.status.metrics.enabled")
+  val METRICS_APP_STATUS_SOURCE_ENABLED =
+    ConfigBuilder("spark.metrics.appStatusSource.enabled")
       .doc("Whether Dropwizard/Codahale metrics " +
         "will be reported for the status of the running spark app.")
       .booleanConf

core/src/main/scala/org/apache/spark/internal/config/package.scala

@@ -638,7 +638,7 @@ package object config {
     .createOptional
 
   private[spark] val METRICS_STATIC_SOURCES_ENABLED =
-    ConfigBuilder("spark.metrics.static.sources.enabled")
+    ConfigBuilder("spark.metrics.staticSources.enabled")
       .doc("Whether to register static sources with the metrics system.")
       .booleanConf
       .createWithDefault(true)

core/src/main/scala/org/apache/spark/status/AppStatusSource.scala

@@ -22,7 +22,7 @@ import AppStatusSource.getCounter
 import com.codahale.metrics.{Counter, Gauge, MetricRegistry}
 
 import org.apache.spark.SparkConf
-import org.apache.spark.internal.config.Status.APP_STATUS_METRICS_ENABLED
+import org.apache.spark.internal.config.Status.METRICS_APP_STATUS_SOURCE_ENABLED
 import org.apache.spark.metrics.source.Source
 
 private [spark] class JobDuration(val value: AtomicLong) extends Gauge[Long] {
@@ -71,7 +71,7 @@ private[spark] object AppStatusSource {
   }
 
   def createSource(conf: SparkConf): Option[AppStatusSource] = {
-    Option(conf.get(APP_STATUS_METRICS_ENABLED))
+    Option(conf.get(METRICS_APP_STATUS_SOURCE_ENABLED))
       .filter(identity)
       .map { _ => new AppStatusSource() }
   }

docs/monitoring.md

@@ -924,7 +924,7 @@ This is the component with the largest amount of instrumented metrics
 
 - namespace=HiveExternalCatalog
   - **note:**: these metrics are conditional to a configuration parameter:
-    `spark.metrics.static.sources.enabled` (default is true) 
+    `spark.metrics.staticSources.enabled` (default is true) 
   - fileCacheHits.count
   - filesDiscovered.count
   - hiveClientCalls.count
@@ -933,7 +933,7 @@ This is the component with the largest amount of instrumented metrics
 
 - namespace=CodeGenerator
   - **note:**: these metrics are conditional to a configuration parameter:
-    `spark.metrics.static.sources.enabled` (default is true) 
+    `spark.metrics.staticSources.enabled` (default is true) 
   - compilationTime (histogram)
   - generatedClassSize (histogram)
   - generatedMethodSize (histogram)
@@ -962,8 +962,8 @@ This is the component with the largest amount of instrumented metrics
   - queue.executorManagement.listenerProcessingTime (timer)
 
 - namespace=appStatus (all metrics of type=counter)
-  - **note:** Introduced in Spark 3.0. Conditional to configuration parameter:  
-   `spark.app.status.metrics.enabled=true` (default is false)
+  - **note:** Introduced in Spark 3.0. Conditional to a configuration parameter:  
+   `spark.metrics.appStatusSource.enabled` (default is false)
   - stages.failedStages.count
   - stages.skippedStages.count
   - stages.completedStages.count
@@ -1057,7 +1057,7 @@ when running in local mode.
 
 - namespace=HiveExternalCatalog
   - **note:**: these metrics are conditional to a configuration parameter:
-    `spark.metrics.static.sources.enabled` (default is true) 
+    `spark.metrics.staticSources.enabled` (default is true) 
   - fileCacheHits.count
   - filesDiscovered.count
   - hiveClientCalls.count
@@ -1066,7 +1066,7 @@ when running in local mode.
 
 - namespace=CodeGenerator
   - **note:**: these metrics are conditional to a configuration parameter:
-    `spark.metrics.static.sources.enabled` (default is true) 
+    `spark.metrics.staticSources.enabled` (default is true) 
   - compilationTime (histogram)
   - generatedClassSize (histogram)
   - generatedMethodSize (histogram)