Metric types are not aligned to public api documentation

[Dynatrace-Jeroen-Hautekeete]

Describe the bug
Dynatrace metric ingestion protocol only supports 2 types : gauge and count - the latter should be submitted as deltas -- there is no separate delta type.
https://docs.dynatrace.com/docs/ingest-from/extend-dynatrace/extend-metrics/reference/metric-ingestion-protocol#payload-format--count

Having an extra type in the SDK is confusing, and on top of that, using the COUNT type is producing undocumented mint lines, which may be rejected on grail.

** Suggestions **
in metric.py :
35: update the COUNT metricType --> COUNT = "count,delta"
[36: remove the DELTA metricType - or keep it with a compatibility warning in the docs ]

67-70: fix the evaluation. only GAUGE should get a "," and COUNT(and DELTA) should get "="

[dlopes7]

I will review the current extensions to see if they are using delta, because that was 100% supported in the past.