[SPARK-12983] [CORE] [DOC] Correct metrics.properties.template

[BenFradet]

There are some typos or plain unintelligible sentences in the metrics template.

[srowen]

While we're here...

• in the master
• , and you can send
• json -> JSON

[srowen]

LGTM, the rest are neutral or small improvements

[SparkQA]

Test build #49999 has finished for PR 10902 at commit 348fd92.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds the following public classes (experimental):
  • # qualified class name (see examples below).

[SparkQA]

Test build #50018 has finished for PR 10902 at commit 9c45b8a.

• This patch fails Spark unit tests.
• This patch merges cleanly.
• This patch adds the following public classes (experimental):
  • # fully qualified class name (see examples below).

[BenFradet]

Unrelated to this pr, triggering a new build.

[BenFradet]

Jenkins, retest this please.

[SparkQA]

Test build #50075 has finished for PR 10902 at commit 9c45b8a.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds the following public classes (experimental):
  • # fully qualified class name (see examples below).

[BenFradet]

cc @srowen and @JoshRosen since this is the spinoff of #10787`

[srowen]

merged to master