[Metricbeat] Change sqs metricset to use Average statistic method

[kaiyan-sheng]

What does this PR do?

This PR is to change sqs metricset to use Average statistic method instead of Sum when querying CloudWatch metrics.
Related Discuss Forum Issue: https://discuss.elastic.co/t/metricbeat-aws-module-dashboard-not-working-and-aws-sqs-messages-visible-multiplied-by-5

This PR also fixed SQS dashboard to show last bucket with "drop_last_bucket":0. This case, when Metricbeat first started, dashboard will not be empty.

Why is it important?

Most of the metrics collected by CloudWatch regarding to SQS is more meaningful with average as statistic method. For example ApproximateAgeOfOldestMessage for the approximate age of the oldest non-deleted message in the queue, and ApproximateNumberOfMessagesVisible for the number of messages available for retrieval from the queue.

Screenshots

This is what I changed for each visualization in SQS dashboard to not drop the last bucket.

[mtojek]

I admit that I had to go over the discuss thread to fully understand the issue. Ship it!

nit: fix is correct, but please re-export JSON files:

Error: failed to parse dashboard from module/aws/_meta/kibana/7/dashboard/Metricbeat-aws-sqs-overview.json: json: cannot unmarshal string into Go struct field .objects.attributes.kibanaSavedObjectMeta.searchSourceJSON of type struct { Index string "json:\"index\"" }