Add validity warning to Scala doc

prometheus-metrics/src/main/scala/org/http4s/metrics/prometheus/Prometheus.scala

@@ -100,6 +100,17 @@ object Prometheus {
       metrics <- createMetricsCollection(registry, prefix, responseDurationSecondsHistogramBuckets)
     } yield createMetricsOps(metrics, Applicative[F].pure(None))
 
+  /** Creates a [[MetricsOps]] that supports Prometheus metrics and records exemplars.
+    *
+    * Warning: The sampler effect is responsible for producing exemplar labels that are valid for the underlying
+    * implementation as errors happening during metric recording will not be handled! For Prometheus version < 1.0,
+    * this means the combined length of keys and values may not exceed 128 characters and the parts must adhere
+    * to the label regex Prometheus defines.
+    *
+    * @param registry a metrics collector registry
+    * @param sampleExemplar an effect that returns the corresponding exemplar labels
+    * @param prefix a prefix that will be added to all metrics
+    */
   def metricsOpsWithExemplars[F[_]: Sync](
       registry: CollectorRegistry,
       sampleExemplar: F[Option[Map[String, String]]],