proposal for supporting aggregation functions for analysis values

Author: bacherfl

text/0097-support-aggregation-functions-in-analysis-value-template.md

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+# KEP 97 Support Aggregation functions in AnalysisValueTemplate
+
+## Motivation
+
+Currently, only the `KeptnMetrics` CRD supports the use of aggregation functions
+(see [here](https://github.com/keptn/lifecycle-toolkit/blob/5fac158a7ffed67f7502fe03683138d717ea1acd/metrics-operator/api/v1beta1/keptnmetric_types.go#L76)).
+Supporting the same aggregation functions might also be useful for `AnalysisValueTemplates` as well, 
+to better support queries that return multiple data points of a time series. 
+This way, the provider returning the results for the AnalysisValueTemplate can check whether
+there are multiple data points in the result for the query and apply the aggregation function in that case.
+
+## Proposed Design
+
+The proposal is to extend the specification of the `AnalysisValueTemplates` with an `spec.aggregation` field,
+similar to what is available in the `KeptnMetric` (see [specification](https://keptn.sh/latest/docs/reference/api-reference/metrics/v1beta1/#rangespec)).
+Using this field, the metrics operator can inspect the result for an analysis value returned by a metrics provider.,
+and, if the aggregation function has been defined, use this function to aggregate the returned result if this is
+an array of multiple values, as opposed to a single value.
+
+## Benefits
+
+- Unified way of defining aggregation functions for query results containing multiple time series data points
+- More flexibility in terms of which queries can be used for each provider
+
+## Open Questions
+
+One argument that might speak against this feature is that usually monitoring provider
+APIs support aggregation functions on their end already,
+such as e.g. [DQL](https://docs.dynatrace.com/docs/platform/grail/dynatrace-query-language/functions#aggregation-functions),
+so it might be sufficient to adapt queries for AnalysisValueTemplates to make use of the monitoring provider's aggregation capabilities.
+
+## References
+
+See the following Github discussion for more context: https://github.com/keptn/lifecycle-toolkit/issues/2650#issuecomment-1906038916