Update RateLimiter.java

[ahmedElatrouz]

Removing the @beta from the RateLimiter

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[ahmedElatrouz]

@googlebot I signed it!

[cgdecker]

Thanks for the PR, but removing @Beta is more of an issue of deciding that we're ok with locking down the API as it is and committing to not making breaking changes. In general we're trying to be more open to that, but I'm not sure about RateLimiter because I believe the person on our team who's worked with it the most feels there are some shortcomings to the design. I'm going to close this PR (we can pretty easily remove the annotation when we decide it should happen) but I'll see if I can get a more informed opinion on #5612.