[FLINK-20533][datadog][release-1.12] Backport flink-metrics-datadog changes from master

[tweise]

I created this PR by copying the files from master because due to the code reformat it would have been cumbersome to cherry pick individual commits. Changes included:

commit 313e20e8e03953a5e1cec9daa467f561ccfbd599Author: Chesnay Schepler chesnay@apache.orgDate:   Tue Dec 8 16:20:29 2020 +0100
    [FLINK-20533][datadog] Add Histogram support
commit 38e424fd5fa7c4a6e6165921689a232a10e85bddAuthor: Chesnay Schepler chesnay@apache.orgDate:   Tue Dec 8 16:16:02 2020 +0100
    [FLINK-20533][datadog] Encapsulate metric meta data        This allows us to use the 'MetricMetaData' container for histograms later on, which will not directly extend 'DMetric'.
commit ea317c20ac4ab76c9e71fb0c1aa8a104e7173057Author: Chesnay Schepler chesnay@apache.orgDate:   Tue Dec 8 14:55:48 2020 +0100
    [hotfix][datadog] Remove outdated comments
commit f01e1916edf015d3f1b800065eeea4c2ae4af7baAuthor: Chesnay Schepler chesnay@apache.orgDate:   Tue Dec 8 16:14:01 2020 +0100
    [hotfix][datadog][tests] Rework tests        - re-parse JSON to ignore order of serialization    - restrict 'DatadogHttpClient.serialize()' to 'DSeries'    - test series serialization    - refer to constants where possible
commit 7fcb628c0cd6a3daf3f19917ef2414d8743bcc7aAuthor: Chesnay Schepler chesnay@apache.orgDate:   Tue Dec 8 14:40:08 2020 +0100
    [hotfix][datadog] Explicitly mark fields relevant for serialization        It was difficult to tell which fields are relevant for serialization. Adding annotations makes this more obvious, and allows us more freedom in regards to naming.
commit 4d5053067199d46134366abc5c8954c57757a82eAuthor: Chesnay Schepler chesnay@apache.orgDate:   Tue Dec 8 14:40:57 2020 +0100
    [hotfix][datadog] Merge DSeries#add*()        The distinction between metric types adds unnecessary complexity.
commit bf058d68acdcdcb4ea28e6fe3a22904a5f385b8aAuthor: Chesnay Schepler chesnay@apache.orgDate:   Tue Dec 8 14:38:44 2020 +0100
    [hotfix][datadog][tests] Cleanup code

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[zentol]

Why are you proposing this backport?

You have missed the documentation updates, but otherwise the backport appears correct.

[tweise]

@zentol we are using Datadog and need the histogram support in 1.12.

I added the missing documentation changes.