Document batch_update/execute_change_ops metric
Document batch_update/execute_change_ops metric

This metric was added by change I8a340d.

Change-Id: I994c93724710cb4efac00b0d4a3f45d22abe5cd9
Signed-off-by: Edwin Kempin <d43fb6c98c97e553b671ed9a9318100f1de386e7@google.com>

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