[SPARK-23096][SS] Migrate rate source to V2

## What changes were proposed in this pull request?

This PR migrate micro batch rate source to V2 API and rewrite UTs to suite V2 test.

## How was this patch tested?

UTs.

Author: jerryshao <sshao@hortonworks.com>

Closes apache#20688 from jerryshao/SPARK-23096.

Author: jerryshao

sql/core/src/main/resources/META-INF/services/org.apache.spark.sql.sources.DataSourceRegister

@@ -5,6 +5,5 @@ org.apache.spark.sql.execution.datasources.orc.OrcFileFormat
 org.apache.spark.sql.execution.datasources.parquet.ParquetFileFormat
 org.apache.spark.sql.execution.datasources.text.TextFileFormat
 org.apache.spark.sql.execution.streaming.ConsoleSinkProvider
-org.apache.spark.sql.execution.streaming.RateSourceProvider
+org.apache.spark.sql.execution.streaming.sources.RateStreamProvider
 org.apache.spark.sql.execution.streaming.sources.TextSocketSourceProvider
-org.apache.spark.sql.execution.streaming.sources.RateSourceProviderV2

sql/core/src/main/scala/org/apache/spark/sql/execution/datasources/DataSource.scala

@@ -41,7 +41,7 @@ import org.apache.spark.sql.execution.datasources.json.JsonFileFormat
 import org.apache.spark.sql.execution.datasources.orc.OrcFileFormat
 import org.apache.spark.sql.execution.datasources.parquet.ParquetFileFormat
 import org.apache.spark.sql.execution.streaming._
-import org.apache.spark.sql.execution.streaming.sources.TextSocketSourceProvider
+import org.apache.spark.sql.execution.streaming.sources.{RateStreamProvider, TextSocketSourceProvider}
 import org.apache.spark.sql.internal.SQLConf
 import org.apache.spark.sql.sources._
 import org.apache.spark.sql.streaming.OutputMode
@@ -566,6 +566,7 @@ object DataSource extends Logging {
     val orc = "org.apache.spark.sql.hive.orc.OrcFileFormat"
     val nativeOrc = classOf[OrcFileFormat].getCanonicalName
     val socket = classOf[TextSocketSourceProvider].getCanonicalName
+    val rate = classOf[RateStreamProvider].getCanonicalName
 
     Map(
       "org.apache.spark.sql.jdbc" -> jdbc,
@@ -587,7 +588,8 @@ object DataSource extends Logging {
       "org.apache.spark.ml.source.libsvm.DefaultSource" -> libsvm,
       "org.apache.spark.ml.source.libsvm" -> libsvm,
       "com.databricks.spark.csv" -> csv,
-      "org.apache.spark.sql.execution.streaming.TextSocketSourceProvider" -> socket
+      "org.apache.spark.sql.execution.streaming.TextSocketSourceProvider" -> socket,
+      "org.apache.spark.sql.execution.streaming.RateSourceProvider" -> rate
     )
   }
 

sql/core/src/main/scala/org/apache/spark/sql/execution/streaming/RateSourceProvider.scala

@@ -24,8 +24,8 @@ import org.json4s.jackson.Serialization
 
 import org.apache.spark.sql.Row
 import org.apache.spark.sql.catalyst.util.DateTimeUtils
-import org.apache.spark.sql.execution.streaming.{RateSourceProvider, RateStreamOffset, ValueRunTimeMsPair}
-import org.apache.spark.sql.execution.streaming.sources.RateStreamSourceV2
+import org.apache.spark.sql.execution.streaming.{RateStreamOffset, ValueRunTimeMsPair}
+import org.apache.spark.sql.execution.streaming.sources.RateStreamProvider
 import org.apache.spark.sql.sources.v2.DataSourceOptions
 import org.apache.spark.sql.sources.v2.reader._
 import org.apache.spark.sql.sources.v2.reader.streaming.{ContinuousDataReader, ContinuousReader, Offset, PartitionOffset}
@@ -40,8 +40,8 @@ class RateStreamContinuousReader(options: DataSourceOptions)
 
   val creationTime = System.currentTimeMillis()
 
-  val numPartitions = options.get(RateStreamSourceV2.NUM_PARTITIONS).orElse("5").toInt
-  val rowsPerSecond = options.get(RateStreamSourceV2.ROWS_PER_SECOND).orElse("6").toLong
+  val numPartitions = options.get(RateStreamProvider.NUM_PARTITIONS).orElse("5").toInt
+  val rowsPerSecond = options.get(RateStreamProvider.ROWS_PER_SECOND).orElse("6").toLong
   val perPartitionRate = rowsPerSecond.toDouble / numPartitions.toDouble
 
   override def mergeOffsets(offsets: Array[PartitionOffset]): Offset = {
@@ -57,12 +57,12 @@ class RateStreamContinuousReader(options: DataSourceOptions)
     RateStreamOffset(Serialization.read[Map[Int, ValueRunTimeMsPair]](json))
   }
 
-  override def readSchema(): StructType = RateSourceProvider.SCHEMA
+  override def readSchema(): StructType = RateStreamProvider.SCHEMA
 
   private var offset: Offset = _
 
   override def setStartOffset(offset: java.util.Optional[Offset]): Unit = {
-    this.offset = offset.orElse(RateStreamSourceV2.createInitialOffset(numPartitions, creationTime))
+    this.offset = offset.orElse(createInitialOffset(numPartitions, creationTime))
   }
 
   override def getStartOffset(): Offset = offset
@@ -98,6 +98,19 @@ class RateStreamContinuousReader(options: DataSourceOptions)
   override def commit(end: Offset): Unit = {}
   override def stop(): Unit = {}
 
+  private def createInitialOffset(numPartitions: Int, creationTimeMs: Long) = {
+    RateStreamOffset(
+      Range(0, numPartitions).map { i =>
+        // Note that the starting offset is exclusive, so we have to decrement the starting value
+        // by the increment that will later be applied. The first row output in each
+        // partition will have a value equal to the partition index.
+        (i,
+          ValueRunTimeMsPair(
+            (i - numPartitions).toLong,
+            creationTimeMs))
+      }.toMap)
+  }
+
 }
 
 case class RateStreamContinuousDataReaderFactory(