[FLINK-38946][flink-metrics] Allow AWS PrivateLink/GCP Cloud Private Service Connect for Datadog integration

[stoiev]

What is the purpose of the change

It aligns the Flink integration with Datadog's supported API host standards, unblocking usage of private networking support in Clouds providers, such as AWS PrivateLink and GCP Private Service Connect Documentation,

Brief change log

In DatadogHttpClient.java, the base URLs should be updated from: https://app.datadoghq.%s/... to https://api.datadoghq.%s/

Verifying this change

(example:)

• TODO: Manually verified the change by running a 4 node cluster with 2 JobManagers and 4 TaskManagers, a stateful streaming program, and killing one JobManager and two TaskManagers during the execution, verifying that recovery happens correctly.

Does this pull request potentially affect one of the following parts:

• Dependencies (does it add or upgrade a dependency): no
• The public API, i.e., is any changed class annotated with @Public(Evolving): no
• The serializers: no
• The runtime per-record code paths (performance sensitive): no
• Anything that affects deployment or recovery: JobManager (and its components), Checkpointing, Kubernetes/Yarn, ZooKeeper: no
• The S3 file system connector: no

[flinkbot]

CI report:

• d501042 Azure: SUCCESS

Bot commands

The @flinkbot bot supports the following commands:

• @flinkbot run azure re-run the last Azure build

[davidradl]

@stoiev I am curious are there scenarios where the old URL can and should be used ? A change like this will break all users of the old URL. I was thinking that you would tolerate multiple endpoints, maybe by indicating in Flink config what type of connection is required then the Flink code can use the appropriate URL.