[Firelens] [request]: Custom tag log line decoration in Firelens

**Tell us about your request**
I want log lines that are routed through Firelens to be decorated with the custom tags from task definitions, clusters, and services.

**Which service(s) is this request for?**
Firelens

**Tell us about the problem you're trying to solve. What are you trying to do, and why is it hard?**
Customers of New Relic Logs want to be able to search their log data using the custom tags that they are already using in ECS. Since these custom tags don't come along with the rest of the ECS metadata, it makes searching log data a lot more difficult.

**Are you currently working around this issue?**
The only work around would be to duplicate these same tags in a custom config file for Fluent Bit, which isn't an ideal solution.

No files, tests, or entry points are mentioned. Start by reviewing the Firelens and ECS integration requirements for task, cluster, and service custom tags; done means routed log lines carry those tags without duplicating them in a Fluent Bit custom configuration.

Written by the indexing model from the issue text.