Set sampling rate in CloudWatch agent

**Is your feature request related to a problem? Please describe.**
A way to set sampling_rate in CloudWatch agent.

[Previous feture_request](https://github.com/aws/amazon-cloudwatch-agent/issues/257) was closed. It was suggested to implement sampling in application_code and leverage the added [filters](https://docs.aws.amazon.com/AmazonCloudWatch/latest/monitoring/CloudWatch-Agent-Configuration-File-Details.html#CloudWatch-Agent-Configuration-File-Logssection) to mimic sampling logs.

**Describe the solution you'd like**
First-class support of setting sampling_rate as part of the [agent config file](https://docs.aws.amazon.com/AmazonCloudWatch/latest/monitoring/CloudWatch-Agent-Configuration-File-Details.html#CloudWatch-Agent-Configuration-File-Complete-Example)

**Describe alternatives you've considered**
Implementing sampling in application_code by flagging sampled/unsampled logs; then instruct CWAgent to only include sampled log_lines via the filters configuration.

**Additional context**
Support setting sampling rate is a common feature in log_systems. E.g. [Sentry](https://docs.sentry.io/platforms/java/guides/log4j2/configuration/sampling/)

Start by reviewing the agent configuration file details and the existing filters configuration described in the issue. Compare the proposed first-class sampling_rate setting with the suggested application-side sampling approach. Done means the agent can accept and apply a sampling rate through its configuration, with the relevant behavior verified by project tests.

Written by the indexing model from the issue text.