[FLINK-36456] feat: Add support to pass Datadog API Key as environment variable

[barryib]

What is the purpose of the change

This pull request adds support for passing the Datadog API Key via an environment variable (DATADOG_API_KEY) instead of only through configuration properties. This enables secure credential management in containerized and cloud-native deployments where environment variables are commonly used.

Brief change log

• Add DATADOG_API_KEY environment variable support in DatadogHttpReporterFactory
• Configuration property apikey takes precedence over environment variable for backward compatibility
• Environment variable provides fallback when no configuration property is provided
• Add comprehensive test suite with 9 test cases covering environment variable support and various configuration scenarios

Resolves FLINK-36456

API Key Resolution Order

The API key is resolved in the following priority order:

1. Configuration property apikey (if provided and non-empty)
2. Environment variable DATADOG_API_KEY (fallback)
3. null (if neither is provided)

Verifying this change

This change added tests and can be verified as follows:

• Added 9 new test cases validating environment variable support:
  • testApiKeyFromConfiguration - Verifies API key from config works
  • testApiKeyFromEnvironmentVariable - Tests environment variable fallback
  • testConfigurationApiKeyTakesPrecedenceOverEnvironmentVariable - Ensures config takes priority
  • testDefaultProxyPort - Validates default proxy port (8080)
  • testCustomProxySettings - Tests custom proxy configuration
  • testDataCenterConfiguration - Verifies EU data center support
  • testMaxMetricsPerRequestConfiguration - Tests custom metrics per request
  • testTagsConfiguration - Validates tag configuration
  • testUseLogicalIdentifierConfiguration - Tests logical identifier flag
• All existing tests remain unchanged and pass
• The change is backward compatible with existing configurations

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

Documentation

• Does this pull request introduce a new feature? yes
• If yes, how is the feature documented? JavaDocs - The getApiKey() method includes comprehensive JavaDoc explaining the resolution order and behavior

[flinkbot]

CI report:

• 00bac62 Azure: FAILURE

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