Implement a throttle transform

Throttle limits the update rate of its input.

src/sensesp/transforms/throttle.h

@@ -0,0 +1,49 @@
+#ifndef SENSESP_SRC_SENSESP_TRANSFORMS_THROTTLE_H_
+#define SENSESP_SRC_SENSESP_TRANSFORMS_THROTTLE_H_
+
+#include <climits>
+#include <elapsedMillis.h>
+
+#include "transform.h"
+
+namespace sensesp {
+
+/**
+ * @brief Throttle the input rate.
+ *
+ * Make sure that a frequently changing value is only
+ * reported at a specified minimum interval. If, for example,
+ * the input value changes every 100ms, but you only want to process it
+ * every 1000ms, you can use this transform. It does not average or buffer
+ * any values, it just throws away values that are too frequent.
+ *
+ * In many cases, you might want to consider the `moving_average` transform
+ * instead of this one. The `moving_average` transform will smooth out
+ * the input values, while this one will only emit the last value.
+ *
+ * @param min_interval Minimum time, in ms, before a new value
+ * is emitted again.
+ *
+ */
+template <typename T>
+class Throttle : public SymmetricTransform<T> {
+ public:
+  Throttle(long min_interval)
+      : SymmetricTransform<T>(), min_interval_{min_interval} {}
+
+  void set(T input, uint8_t inputChannel = 0) override {
+    if (age_ < min_interval_) {
+      return;
+    }
+    age_ = 0;
+    this->emit(input);
+  }
+
+ protected:
+  long min_interval_;
+  elapsedMillis age_ = LONG_MAX;
+};
+
+}  // namespace sensesp
+
+#endif  // SENSESP_SRC_SENSESP_TRANSFORMS_THROTTLE_H_