Fix HDR and DDS aggregation functions

[mfelsche]

Pull request

Description

Both were issueing invalid numbers on zero elements and were swallowing events when receiving > 8192 elements.

Related

• Related Issues:
  • fixes stats::dds does not include sum #587
  • fixes Issue with aggregation for high throughput volume metrics - resulting in incorrect aggregate value #1242
  • fixes Tremor aggregation function "stats::dds" produces incorrect values when count is zero  #546

Checklist

• The RFC, if required, has been submitted and approved
• Any user-facing impact of the changes is reflected in docs.tremor.rs
• The code is tested
• Use of unsafe code is reasoned about in a comment
• Update CHANGELOG.md appropriately, recording any changes, bug fixes, or other observable changes in behavior
• The performance impact of the change is measured (see below)

Performance

Didn't test it.

[darach]

LGTM 👌