Ability to reload environment variables

We have a use case whereby (system) environment variables are created in one session managed by the ssm-agent, and consumed in another, later on.
We have found that unless we restart the agent, the environment variables are not available in later sessions through the (session) environment provided by the agent.
It would be great if we could trigger an environment variable reload or something of the like to permit environment variable use across sessions.

No files or tests are named in the issue. Start by tracing how the agent creates session environments and when system environment variables are read; determine how a later session could observe variables created earlier without restarting the agent. Done means a supported reload or equivalent makes those variables available in later sessions, with coverage for the cross-session behavior.

Written by the indexing model from the issue text.