[SPARK-24748][SS][FOLLOWUP] Switch custom metrics to Unstable APIs

[arunmahadevan]

• Mark custom metrics related APIs as unstable
• Move CustomMetrics (and a few other streaming interfaces in parent package) to streaming package

Ideally could move v2/reader/streaming and v2/writer/streaming under streaming/reader and streaming/writer but that can be a follow up PR if required.

How was this patch tested?

Existing unit tests

Please review http://spark.apache.org/contributing.html before opening a pull request.

[jerryshao]

Seems there's another similar PR #22296 .

[HyukjinKwon]

Thanks, @jerryshao for pointing this out. I will close mine after we see what we want.

[SparkQA]

Test build #95521 has finished for PR 22299 at commit 49a94c6.

• This patch fails due to an unknown error code, -9.
• This patch merges cleanly.
• This patch adds no public classes.

[HyukjinKwon]

retest this please

[SparkQA]

Test build #95530 has finished for PR 22299 at commit 49a94c6.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.

[HyukjinKwon]

Let's close this.