Feature request: ability to track number of requests made

[extemporalgenome]

Related to controlling pagination via https://github.com/guregu/dynamo/issues/231, it would be useful to known how many top-level DynamoDB requests were made as part of a high-level operation. Typically this would be for queries/scans (how many sequential pages were fetched?), though potentially it could also apply to BatchWriteItem or BatchReadItem calls.

Perhaps this could live in ConsumedCapacity.

It's unclear if there's value in distinguishing between planned-for requests and actual requests, in order to account for retries due to throttling, etc. Retries could also be tracked explicitly, of course.