## What changes were proposed in this pull request?
class `org.apache.spark.sql.types.Metadata` is widely used in mllib to store some ml attributes. `Metadata` is commonly stored in `Alias` expression.
```
case class Alias(child: Expression, name: String)(
val exprId: ExprId = NamedExpression.newExprId,
val qualifier: Option[String] = None,
val explicitMetadata: Option[Metadata] = None,
override val isGenerated: java.lang.Boolean = false)
```
The `Metadata` can take a big memory footprint since the number of attributes is big ( in scale of million). When `toJSON` is called on `Alias` expression, the `Metadata` will also be converted to a big JSON string.
If a plan contains many such kind of `Alias` expressions, it may trigger out of memory error when `toJSON` is called, since converting all `Metadata` references to JSON will take huge memory.
With this PR, we will skip scanning Metadata when doing JSON conversion. For a reproducer of the OOM, and analysis, please look at jira https://issues.apache.org/jira/browse/SPARK-17356.
## How was this patch tested?
Existing tests.
Author: Sean Zhong <seanzhong@databricks.com>
Closes#14915 from clockfly/json_oom.
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