Add docs for generic SLO checker

docs/creating-an-execution.md

@@ -58,7 +58,29 @@ As a Benchmark may define multiple supported load and resource types, an Executi
 ## Definition of SLOs
 
 SLOs provide a way to quantify whether a certain load intensity can be handled by a certain amount of provisioned resources.
-An Execution must at least specify one SLO to be checked.
+In Theodolite, SLO are evaluated by requesting monitoring data from Prometheus and analyzing it in a benchmark-specific way.
+An Execution must at least define one SLO to be checked.
+
+A good choice to get started is defining an SLO of type `generic`:
+
+```yaml
+- sloType: "generic"
+  prometheusUrl: "http://prometheus-operated:9090"
+  offset: 0
+  properties:
+    externalSloUrl: "http://localhost:8082"
+    promQLQuery: "sum by(job) (kafka_streams_stream_task_metrics_dropped_records_total>=0)"
+    warmup: 60 # in seconds
+    queryAggregation: max
+    repetitionAggregation: median
+    operator: lte
+    threshold: 1000
+```
+
+All you have to do is to define a [PromQL query](https://prometheus.io/docs/prometheus/latest/querying/basics/) describing which metrics should be requested (`promQLQuery`) and how the resulting time series should be evaluated. With `queryAggregation` you specify how the resulting time series is aggregated to a single value and `repetitionAggregation` describes how the results of multiple repetitions are aggregated. Possible values are
+`mean`, `median`, `mode`, `sum`, `count`, `max`, `min`, `std`, `var`, `skew`, `kurt` as well as percentiles such as `p99` or `p99.9`. The result of aggregation all repetitions is checked against `threshold`. This check is performed using an `operator`, which describes that the result must be "less than" (`lt`), "less than equal" (`lte`), "greater than" (`gt`) or "greater than equal" (`gte`) to the threshold.
+
+In case you need to evaluate monitoring data in a more flexible fashion, you can also change the value of `externalSloUrl` to your custom SLO checker. Have a look at the source code of the [generic SLO checker](https://github.com/cau-se/theodolite/tree/master/slo-checker/generic) to get started.
 
 ## Experimental Setup
 
@@ -72,7 +94,7 @@ The experimental setup can be configured by:
 
 ## Configuration Overrides
 
-In cases where only small modifications of a system under test should be benchmarked, it is not necessarily required to [create a new benchmark](creating-a-benchmark).
+In cases where only small modifications of a system under test should be benchmarked, it is not necessary to [create a new benchmark](creating-a-benchmark).
 Instead, also Executions allow to do small reconfigurations, such as switching on or off a specific Pod scheduler.
 
 This is done by defining `configOverrides` in the Execution. Each override consists of a patcher, defining which Kubernetes resource should be patched in which way, and a value the patcher is applied with.