[ECS] [request]: Support Mem_Buf_Limit in FireLens

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**Tell us about your request**

With FireLens, the[ input definitions for Fluent Bit are generated by ECS](https://aws.amazon.com/blogs/containers/under-the-hood-firelens-for-amazon-ecs-tasks/). This prevents customers from setting any custom options on the input configuration. `Mem_Buf_Limit` is an input configuration option which sets the total memory available for buffering logs.

This field should probably be optionally configurable by customers. We should determine if there are any other input options as well which might need to be configured.

UPDATE: We will likely work with Fluent Bit upstream to contribute this instead: https://github.com/fluent/fluent-bit/discussions/5711

**Which service(s) is this request for?**
ECS EC2, and ECS Fargate

**Are you currently working around this issue?**

There's a way to "hack" in input configuration, which is not ideal but I should possibly publish a tutorial on if it is desired...

**EDIT:** Here is the detailed tutorial: https://aws.amazon.com/blogs/containers/how-to-set-fluentd-and-fluent-bit-input-parameters-in-firelens/

Read the FireLens input-generation description and the linked Fluent Bit discussion first; the issue does not identify a file or test in this roadmap repository. Confirm the upstream approach and whether ECS EC2 and Fargate support is delivered; done is support for configurable Mem_Buf_Limit and any agreed input options, rather than a roadmap-only change.

Written by the indexing model from the issue text.