Add Prometheus text format serializers.

exporters/prometheus/src/main/java/io/opentelemetry/exporter/prometheus/MetricAdapter.java

@@ -98,7 +98,7 @@ static Collector.Type toMetricFamilyType(MetricData metricData) {
     return Collector.Type.UNKNOWN;
   }
 
-  private static final Function<String, String> sanitizer = new LabelNameSanitizer();
+  static final Function<String, String> sanitizer = new LabelNameSanitizer();
 
   // Converts a list of points from MetricData to a list of Prometheus Samples.
   static List<Sample> toSamples(
@@ -277,7 +277,7 @@ private static int estimateNumSamples(int numPoints, MetricDataType type) {
     return numPoints;
   }
 
-  private static Collection<? extends PointData> getPoints(MetricData metricData) {
+  static Collection<? extends PointData> getPoints(MetricData metricData) {
     switch (metricData.getType()) {
       case DOUBLE_GAUGE:
         return metricData.getDoubleGaugeData().getPoints();

exporters/prometheus/src/main/java/io/opentelemetry/exporter/prometheus/PrometheusType.java

@@ -0,0 +1,59 @@
+/*
+ * Copyright The OpenTelemetry Authors
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+package io.opentelemetry.exporter.prometheus;
+
+import io.opentelemetry.sdk.metrics.data.AggregationTemporality;
+import io.opentelemetry.sdk.metrics.data.DoubleSumData;
+import io.opentelemetry.sdk.metrics.data.LongSumData;
+import io.opentelemetry.sdk.metrics.data.MetricData;
+
+// Four types we use are same in prometheus and openmetrics format
+enum PrometheusType {
+  GAUGE("gauge"),
+  COUNTER("counter"),
+  SUMMARY("summary"),
+  HISTOGRAM("histogram");
+
+  private final String typeString;
+
+  PrometheusType(String typeString) {
+    this.typeString = typeString;
+  }
+
+  static PrometheusType forMetric(MetricData metric) {
+    switch (metric.getType()) {
+      case LONG_GAUGE:
+      case DOUBLE_GAUGE:
+        return GAUGE;
+      case LONG_SUM:
+        LongSumData longSumData = metric.getLongSumData();
+        if (longSumData.isMonotonic()
+            && longSumData.getAggregationTemporality() == AggregationTemporality.CUMULATIVE) {
+          return COUNTER;
+        }
+        return GAUGE;
+      case DOUBLE_SUM:
+        DoubleSumData doubleSumData = metric.getDoubleSumData();
+        if (doubleSumData.isMonotonic()
+            && doubleSumData.getAggregationTemporality() == AggregationTemporality.CUMULATIVE) {
+          return COUNTER;
+        }
+        return GAUGE;
+      case SUMMARY:
+        return SUMMARY;
+      case HISTOGRAM:
+      case EXPONENTIAL_HISTOGRAM:
+        return HISTOGRAM;
+    }
+    throw new IllegalArgumentException(
+        "Unsupported metric type, this generally indicates version misalignment "
+            + "among opentelemetry dependencies. Please make sure to use opentelemetry-bom.");
+  }
+
+  String getTypeString() {
+    return typeString;
+  }
+}