Indicate max_concurrency for throughput and disallow running standalone without --rate

[sjmonson]

Summary

Add a indicator to throughput similar to concurrent's concurrent@CONC. Additionally require specifying --rate for throughput when running standalone rather than defaulting to MAX_CONCURRENCY.

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Use of AI

• Includes AI-assisted code completion
• Includes code generated by an AI application
• Includes AI-generated tests (NOTE: AI written tests should have a docstring that includes ## WRITTEN BY AI ##)

[jaredoconnell]

This works for me if this is the direction we want to go. I like that it specifies the max concurrency in the identifier. And I do prefer the value be passed in there rather than with an env variable.

[markurtz]

LGTM