diff --git a/core/src/main/java/dev/faststats/core/data/ArrayMetric.java b/core/src/main/java/dev/faststats/core/data/ArrayMetric.java
index bcba91a5..4a6e027b 100644
--- a/core/src/main/java/dev/faststats/core/data/ArrayMetric.java
+++ b/core/src/main/java/dev/faststats/core/data/ArrayMetric.java
@@ -8,7 +8,7 @@
 import java.util.concurrent.Callable;
 
 final class ArrayMetric<T> extends SimpleMetric<T[]> {
-    public ArrayMetric(@SourceId final String id, final Callable<T @Nullable []> callable) throws IllegalArgumentException {
+    public ArrayMetric(@SourceId final String id, final Callable<? extends T @Nullable []> callable) throws IllegalArgumentException {
         super(id, callable);
     }
 
diff --git a/core/src/main/java/dev/faststats/core/data/MapMetric.java b/core/src/main/java/dev/faststats/core/data/MapMetric.java
new file mode 100644
index 00000000..c9a5e80e
--- /dev/null
+++ b/core/src/main/java/dev/faststats/core/data/MapMetric.java
@@ -0,0 +1,28 @@
+package dev.faststats.core.data;
+
+import com.google.gson.JsonElement;
+import com.google.gson.JsonObject;
+import org.jspecify.annotations.Nullable;
+
+import java.util.Map;
+import java.util.Optional;
+import java.util.concurrent.Callable;
+
+final class MapMetric<T> extends SimpleMetric<Map<String, ? extends T>> {
+    public MapMetric(@SourceId final String id, final Callable<? extends @Nullable Map<String, ? extends T>> callable) throws IllegalArgumentException {
+        super(id, callable);
+    }
+
+    @Override
+    public Optional<JsonElement> getData() throws Exception {
+        return compute().map(data -> {
+            final var object = new JsonObject();
+            data.forEach((key, value) -> {
+                if (value instanceof final Boolean bool) object.addProperty(key, bool);
+                else if (value instanceof final Number number) object.addProperty(key, number);
+                else object.addProperty(key, value.toString());
+            });
+            return object;
+        });
+    }
+}
diff --git a/core/src/main/java/dev/faststats/core/data/Metric.java b/core/src/main/java/dev/faststats/core/data/Metric.java
index 7fb753cb..40ee7830 100644
--- a/core/src/main/java/dev/faststats/core/data/Metric.java
+++ b/core/src/main/java/dev/faststats/core/data/Metric.java
@@ -4,6 +4,7 @@
 import org.jetbrains.annotations.Contract;
 import org.jspecify.annotations.Nullable;
 
+import java.util.Map;
 import java.util.Optional;
 import java.util.concurrent.Callable;
 
@@ -33,7 +34,7 @@ public interface Metric<T> {
      * @since 0.16.0
      */
     @Contract(pure = true)
-    Optional<T> compute() throws Exception;
+    Optional<? extends T> compute() throws Exception;
 
     /**
      * Get the metric data as a JSON element.
@@ -96,6 +97,54 @@ static Metric<Number[]> numberArray(@SourceId final String id, final Callable<Nu
         return new ArrayMetric<>(id, callable);
     }
 
+    /**
+     * Create a string map metric.
+     *
+     * @param id       the source id
+     * @param callable the metric data callable
+     * @return the string map metric
+     * @throws IllegalArgumentException if the source id is invalid
+     * @apiNote The callable must be thread-safe and pure (i.e. not modify any shared state).
+     * @see #compute()
+     * @since 0.23.0
+     */
+    @Contract(value = "_, _ -> new", pure = true)
+    static Metric<Map<String, ? extends String>> stringMap(@SourceId final String id, final Callable<? extends @Nullable Map<String, String>> callable) throws IllegalArgumentException {
+        return new MapMetric<>(id, callable);
+    }
+
+    /**
+     * Create a boolean map metric.
+     *
+     * @param id       the source id
+     * @param callable the metric data callable
+     * @return the boolean map metric
+     * @throws IllegalArgumentException if the source id is invalid
+     * @apiNote The callable must be thread-safe and pure (i.e. not modify any shared state).
+     * @see #compute()
+     * @since 0.23.0
+     */
+    @Contract(value = "_, _ -> new", pure = true)
+    static Metric<Map<String, ? extends Boolean>> booleanMap(@SourceId final String id, final Callable<? extends @Nullable Map<String, Boolean>> callable) throws IllegalArgumentException {
+        return new MapMetric<>(id, callable);
+    }
+
+    /**
+     * Create a number map metric.
+     *
+     * @param id       the source id
+     * @param callable the metric data callable
+     * @return the number map metric
+     * @throws IllegalArgumentException if the source id is invalid
+     * @apiNote The callable must be thread-safe and pure (i.e. not modify any shared state).
+     * @see #compute()
+     * @since 0.23.0
+     */
+    @Contract(value = "_, _ -> new", pure = true)
+    static Metric<Map<String, ? extends Number>> numberMap(@SourceId final String id, final Callable<? extends @Nullable Map<String, ? extends Number>> callable) throws IllegalArgumentException {
+        return new MapMetric<>(id, callable);
+    }
+
     /**
      * Create a metric for a boolean value.
      *
diff --git a/core/src/main/java/dev/faststats/core/data/SimpleMetric.java b/core/src/main/java/dev/faststats/core/data/SimpleMetric.java
index 8d635e46..e3f55476 100644
--- a/core/src/main/java/dev/faststats/core/data/SimpleMetric.java
+++ b/core/src/main/java/dev/faststats/core/data/SimpleMetric.java
@@ -8,9 +8,9 @@
 
 abstract class SimpleMetric<T> implements Metric<T> {
     private final @SourceId String id;
-    private final Callable<@Nullable T> callable;
+    private final Callable<? extends @Nullable T> callable;
 
-    public SimpleMetric(@SourceId final String id, final Callable<@Nullable T> callable) throws IllegalArgumentException {
+    public SimpleMetric(@SourceId final String id, final Callable<? extends @Nullable T> callable) throws IllegalArgumentException {
         if (!id.matches(SourceId.PATTERN)) {
             throw new IllegalArgumentException("Invalid source id '" + id + "', must match '" + SourceId.PATTERN + "'");
         }
@@ -28,7 +28,7 @@ public final Optional<T> compute() throws Exception {
     }
 
     @Override
-    public boolean equals(final Object o) {
+    public boolean equals(@Nullable final Object o) {
         if (o == null || getClass() != o.getClass()) return false;
         final SimpleMetric<?> that = (SimpleMetric<?>) o;
         return Objects.equals(id, that.id);
diff --git a/core/src/main/java/dev/faststats/core/data/SingleValueMetric.java b/core/src/main/java/dev/faststats/core/data/SingleValueMetric.java
index 458679ce..cbfc7e69 100644
--- a/core/src/main/java/dev/faststats/core/data/SingleValueMetric.java
+++ b/core/src/main/java/dev/faststats/core/data/SingleValueMetric.java
@@ -8,7 +8,7 @@
 import java.util.concurrent.Callable;
 
 final class SingleValueMetric<T> extends SimpleMetric<T> {
-    public SingleValueMetric(@SourceId final String id, final Callable<@Nullable T> callable) throws IllegalArgumentException {
+    public SingleValueMetric(@SourceId final String id, final Callable<? extends @Nullable T> callable) throws IllegalArgumentException {
         super(id, callable);
     }
 
diff --git a/gradle.properties b/gradle.properties
index 1df5d313..4433e215 100644
--- a/gradle.properties
+++ b/gradle.properties
@@ -1 +1 @@
-version=0.22.1
+version=0.23.0