[processor/cumulativetodelta] Convert cumulative Histograms to delta temporality (#12423)

processor/cumulativetodeltaprocessor/README.md

@@ -11,6 +11,8 @@
 
 The cumulative to delta processor (`cumulativetodeltaprocessor`) converts monotonic, cumulative sum and histogram metrics to monotonic, delta metrics. Non-monotonic sums and exponential histograms are excluded.
 
+Histogram conversion is currently behind a [feature gate](#feature-gate-configurations) and will only be converted if the feature flag is set.
+
 ## Configuration
 
 Configuration is specified through a list of metrics. The processor uses metric names to identify a set of cumulative metrics and converts them from cumulative to delta.
@@ -76,6 +78,14 @@ processors:
         # convert all cumulative sum or histogram metrics to delta
 ```
 
+## Feature gate configurations
+
+The **processor.cumulativetodeltaprocessor.EnableHistogramSupport** feature flag controls whether cumulative histograms will be converted to delta temporality or not. It is disabled by default, meaning histograms will not be modified by the processor.
+
+Pass `--feature-gates processor.cumulativetodeltaprocessor.EnableHistogramSupport` to enable this feature.
+
+This feature flag will be removed, and histograms will be enabled by default in release v0.60.0, September 2022.
+
 ## Warnings
 
 - [Statefulness](https://github.com/open-telemetry/opentelemetry-collector/blob/main/docs/standard-warnings.md#statefulness): The cumulativetodelta processor's calculates delta by remembering the previous value of a metric.  For this reason, the calculation is only accurate if the metric is continuously sent to the same instance of the collector.  As a result, the cumulativetodelta processor may not work as expected if used in a deployment of multiple collectors.  When using this processor it is best for the data source to being sending data to a single collector.