Cache toString() for MetricKey and MetricName in instance variables

[FuRyanf]

This PR addresses the performance overhead caused by repeated string concatenation in the toString() method of the MetricName and MetricKey classes. The issue was identified as a significant contributor to CPU usage during metric processing in Beam’s Samza integration.

To mitigate this, the toString() results for MetricName and MetricKey are now cached. By storing the computed string representation on first access, we eliminate redundant concatenation operations in subsequent calls. This change reduces the computational cost associated with metrics processing, leading to improved performance in scenarios with frequent metric updates.

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[xinyuiscool]

It seems quite unusual to add filed to a AutoValue. Instead, I think you can try this: https://github.com/google/auto/blob/main/value/userguide/howto.md#memoize_hash_tostring

Btw, can we also put this patch against open source? It's kind of core change, and oss have a lot of tests running against it.

Thanks.