Add Datadog metrics listener

[Meenakshi2000]

Summary

This PR adds optional Datadog metrics and tracing support to Locust Grasshopper.

The implementation:

• Adds a Datadog listener that can publish Locust metrics to Datadog.
• Supports OpenTelemetry tracing for Locust requests and custom checks.
• Keeps Datadog functionality optional so existing users are unaffected unless the required configuration is provided.
• Maintains backwards compatibility with existing InfluxDB and other metric backends.

Changes

• Add Datadog metrics listener.
• Add OpenTelemetry tracing support.
• Add helper APIs for emitting custom metrics from downstream projects.
• Normalize check names and metric tags for Datadog.
• Update documentation with Datadog configuration and usage examples.

Backward Compatibility

No existing functionality is changed by default.

If Datadog is not configured:

• Existing Locust execution is unchanged.
• Existing metric backends continue to work.
• Existing users do not need to make any configuration changes.

Validation

Validated the following scenarios:

• Existing Locust execution without Datadog configuration behaves exactly as before.
• Datadog metrics are emitted when Datadog configuration is provided.
• OpenTelemetry traces are generated for supported requests and checks.
• Existing metric backends continue to function correctly alongside the Datadog integration.

Related

This PR is consumed by Shield in:

• TWYX-806 – Send metrics and traces to Datadog