Fix Micrometer registry serialization error due to failed meter

[eyalkoren]

What does this PR do?

When a Micrometer meter throws Exception while trying to retrieve some meter value, the serialization of the entire metric registry will fail. If the failure is consistent, all meters of this registry would never be reported.
With this PR, a meter can fail only once, omitting a single warning, without failing the serialization of the registry.

In addition, in an attempt to fix at least one type of errors related to querying meter values, I tried to set the meter's class loader as the thread's context CL for the time of query.
NOTE FOR REVIEWER I think it's lightweight enough to do for each meter, but we can also have two caches of failed meters, so that we don't set context CL by default, try to re-execute meter query after the first failure with the context CL and if succeeds, add it to a different cache. If the second attempt fails, we can then add to the generally-disabled meters.

Checklist

• This is a bugfix
  • I have updated CHANGELOG.asciidoc
  • I have added tests that would fail without this fix

[apmmachine]

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Build stats

• Build Cause: Pull request #1741 updated

• Start Time: 2021-04-19T08:03:40.797+0000

• Duration: 59 min 39 sec

• Commit: 70012cb

Test stats 🧪

Test	Results
Failed	0
Passed	1989
Skipped	19
Total	2008

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Test stats 🧪

Test	Results
Failed	0
Passed	1989
Skipped	19
Total	2008