use batch values API for CloudWatch PutMetric data call

[taraspos]

Some time ago, AWS introduced the way to publish a batch of metric values in one call.

So there is no point in sending last(values) if we could send the whole batch.
This PR implements the sending of values batch,

[peterbourgon]

panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x0 pc=0x900100]

goroutine 28 [running]:
github.com/go-kit/kit/metrics/cloudwatch.(*mockCloudWatch).PutMetricData(0xc0001d0900, 0xc00000e080, 0x0, 0x0, 0x0)
	/home/build/go/src/github.com/go-kit/kit/metrics/cloudwatch/cloudwatch_test.go:36 +0x150
github.com/go-kit/kit/metrics/cloudwatch.(*CloudWatch).Send.func5(0xc00019de60, 0xc000090840, 0xc00008e220, 0x1, 0x1)
	/home/build/go/src/github.com/go-kit/kit/metrics/cloudwatch/cloudwatch.go:251 +0x233
created by github.com/go-kit/kit/metrics/cloudwatch.(*CloudWatch).Send
	/home/build/go/src/github.com/go-kit/kit/metrics/cloudwatch/cloudwatch.go:246 +0x55a
FAIL	github.com/go-kit/kit/metrics/cloudwatch	0.025s

[taraspos]

panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x0 pc=0x900100]

goroutine 28 [running]:
github.com/go-kit/kit/metrics/cloudwatch.(*mockCloudWatch).PutMetricData(0xc0001d0900, 0xc00000e080, 0x0, 0x0, 0x0)
	/home/build/go/src/github.com/go-kit/kit/metrics/cloudwatch/cloudwatch_test.go:36 +0x150
github.com/go-kit/kit/metrics/cloudwatch.(*CloudWatch).Send.func5(0xc00019de60, 0xc000090840, 0xc00008e220, 0x1, 0x1)
	/home/build/go/src/github.com/go-kit/kit/metrics/cloudwatch/cloudwatch.go:251 +0x233
created by github.com/go-kit/kit/metrics/cloudwatch.(*CloudWatch).Send
	/home/build/go/src/github.com/go-kit/kit/metrics/cloudwatch/cloudwatch.go:246 +0x55a
FAIL	github.com/go-kit/kit/metrics/cloudwatch	0.025s

Yup, sorry.
This is a problem in the tests, they are not ready for the absence of Value field.
Will fix!

[taraspos]

@peterbourgon hey, fixing the tests took some effort and changes.
One way was just to take the latest value from the array of values for CloudWatch, but that doesn't seem right, so I had to change the tests to accept the array of values, could you please review and give your feedback, please?

[peterbourgon]

@cam-stitt This is your package :D Feedback?

@trane9991 Did you see package cloudwatch2? Should this be ported there, too?

[taraspos]

hmmm... I see that cloudwatch2 uses the StatisticsSet which allows pushing already calculated values, this is a different approach, not sure which one is better yet :)

[cam-stitt]

I like the implementation, but I think we can just tidy it a little and always use the batch request.

[cam-stitt]

We could move this above the datum initialization to save an alloc, but not a big deal.

[peterbourgon]

Nice, thanks!