otlp: Add DeltaTemporalitySelector (#1568)

Co-authored-by: Cijo Thomas <[email protected]>

opentelemetry-otlp/CHANGELOG.md

@@ -2,6 +2,12 @@
 
 ## vNext
 
+### Added
+
+- Added `DeltaTemporalitySelector` ([#1568])
+
+[#1568]: https://github.com/open-telemetry/opentelemetry-rust/pull/1568
+
 ## v0.15.0
 
 ### Changed

opentelemetry-otlp/src/metric.rs

@@ -169,6 +169,16 @@ where
         }
     }
 
+    /// Build with delta temporality selector.
+    ///
+    /// This temporality selector is equivalent to OTLP Metrics Exporter's
+    /// `Delta` temporality preference (see [its documentation][exporter-docs]).
+    ///
+    /// [exporter-docs]: https://github.com/open-telemetry/opentelemetry-specification/blob/a1c13d59bb7d0fb086df2b3e1eaec9df9efef6cc/specification/metrics/sdk_exporters/otlp.md#additional-configuration
+    pub fn with_delta_temporality(self) -> Self {
+        self.with_temporality_selector(DeltaTemporalitySelector)
+    }
+
     /// Build with the given aggregation selector
     pub fn with_aggregation_selector<T: AggregationSelector + 'static>(self, selector: T) -> Self {
         OtlpMetricPipeline {
@@ -248,6 +258,35 @@ impl<RT, EB: Debug> Debug for OtlpMetricPipeline<RT, EB> {
     }
 }
 
+/// A temporality selector that returns [`Delta`][Temporality::Delta] for all
+/// instruments except `UpDownCounter` and `ObservableUpDownCounter`.
+///
+/// This temporality selector is equivalent to OTLP Metrics Exporter's
+/// `Delta` temporality preference (see [its documentation][exporter-docs]).
+///
+/// [exporter-docs]: https://github.com/open-telemetry/opentelemetry-specification/blob/a1c13d59bb7d0fb086df2b3e1eaec9df9efef6cc/specification/metrics/sdk_exporters/otlp.md#additional-configuration
+#[derive(Debug)]
+struct DeltaTemporalitySelector;
+
+impl TemporalitySelector for DeltaTemporalitySelector {
+    #[rustfmt::skip]
+    fn temporality(&self, kind: InstrumentKind) -> Temporality {
+        match kind {
+            InstrumentKind::Counter
+            | InstrumentKind::Histogram
+            | InstrumentKind::ObservableCounter
+            | InstrumentKind::Gauge
+            | InstrumentKind::ObservableGauge => {
+                Temporality::Delta
+            }
+            InstrumentKind::UpDownCounter
+            | InstrumentKind::ObservableUpDownCounter => {
+                Temporality::Cumulative
+            }
+        }
+    }
+}
+
 /// An interface for OTLP metrics clients
 #[async_trait]
 pub trait MetricsClient: fmt::Debug + Send + Sync + 'static {