[SPARK-21409][SS] Follow up PR to allow different types of custom metrics to be exposed

[tdas]

What changes were proposed in this pull request?

Implementation may expose both timing as well as size metrics. This PR enables that.

[zsxwing]

LGTM pending tests

[SparkQA]

Test build #79690 has finished for PR 18661 at commit 351c207.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds the following public classes (experimental):
• trait StateStoreCustomMetric
• case class StateStoreCustomSizeMetric(name: String, desc: String) extends StateStoreCustomMetric
• case class StateStoreCustomTimingMetric(name: String, desc: String) extends StateStoreCustomMetric

[tdas]

Merging to master.