Replace ecs-metadata with awsecscontainermetrics

[pjanotti]

Requirements

• Content follows Splunk guidelines for style and formatting.
• You are contributing original content.

Describe the change

Update the docs to deprecate ecs-metadata and replace it with the awsecscontainermetrics.

Corresponding change on the Splunk Collector: signalfx/splunk-otel-collector#5541

[aurbiztondo-splunk]

Thanks for the updates, @pjanotti!