[SPARK-55746][SQL][TESTS] Fix unable to load custom metric object SupportedV1WriteMetric

[pan3793]

What changes were proposed in this pull request?

Bug was introduced by SPARK-50315 (#48867), won't fail the test, just causes lots of warning logs

$ build/sbt "sql/testOnly *V1WriteFallbackSuite"
...
18:06:25.108 WARN org.apache.spark.sql.execution.ui.SQLAppStatusListener: Unable to load custom metric object for class `org.apache.spark.sql.connector.SupportedV1WriteMetric`. Please make sure that the custom metric class is in the classpath and it has 0-arg constructor.
org.apache.spark.SparkException: org.apache.spark.sql.connector.SupportedV1WriteMetric did not have a zero-argument constructor or a single-argument constructor that accepts SparkConf. Note: if the class is defined inside of another Scala class, then its constructors may accept an implicit parameter that references the enclosing class; in this case, you must define the class as a top-level class in order to prevent this extra parameter from breaking Spark's ability to find a valid constructor.
	at org.apache.spark.util.Utils$.$anonfun$loadExtensions$1(Utils.scala:2871)
	at scala.collection.immutable.List.flatMap(List.scala:283)
	at scala.collection.immutable.List.flatMap(List.scala:79)
	at org.apache.spark.util.Utils$.loadExtensions(Utils.scala:2853)
	at org.apache.spark.sql.execution.ui.SQLAppStatusListener.$anonfun$aggregateMetrics$3(SQLAppStatusListener.scala:220)
	at scala.Option.map(Option.scala:242)
	at org.apache.spark.sql.execution.ui.SQLAppStatusListener.$anonfun$aggregateMetrics$2(SQLAppStatusListener.scala:214)
... (repeat many times)

Why are the changes needed?

Fix UT.

Does this PR introduce any user-facing change?

No.

How was this patch tested?

Verified locally, no warnings printed after fixing.

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

No.

[dongjoon-hyun]

+1, LGTM.

cc @olaky and @cloud-fan

• #48867

[dongjoon-hyun]

Since this is a test PR, I backported this to master/4.1/4.0 to help the testing Spark 4.x.