[dynamic control] Add trace sampling validation (#2604)

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

Author: jackshirazi

dynamic-control/src/main/java/io/opentelemetry/contrib/dynamic/policy/PolicyValidator.java

@@ -0,0 +1,56 @@
+/*
+ * Copyright The OpenTelemetry Authors
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+package io.opentelemetry.contrib.dynamic.policy;
+
+import javax.annotation.Nullable;
+
+public interface PolicyValidator {
+  /**
+   * Validates a policy configuration provided as a JSON string.
+   *
+   * @param json The JSON string containing the policy configuration.
+   * @return The validated {@link TelemetryPolicy}, or {@code null} if the JSON does not contain a
+   *     valid policy for this validator.
+   */
+  @Nullable
+  TelemetryPolicy validate(String json);
+
+  /**
+   * Returns the type of the policy this validator handles.
+   *
+   * @return The policy type string (e.g., "trace-sampling").
+   */
+  String getPolicyType();
+
+  /**
+   * Validates a policy configuration provided as a key-value pair (alias).
+   *
+   * <p>This is intended for simple configuration cases where a full JSON object is not necessary.
+   *
+   * @param key The alias key (e.g., "trace-sampling.probability").
+   * @param value The value associated with the key.
+   * @return The validated {@link TelemetryPolicy}, or {@code null} if the key/value pair is invalid
+   *     or not handled by this validator.
+   */
+  @Nullable
+  default TelemetryPolicy validateAlias(String key, String value) {
+    throw new UnsupportedOperationException(
+        "Alias validation is not supported by validator "
+            + getClass().getName()
+            + " for key "
+            + key);
+  }
+
+  /**
+   * Returns the alias key supported by this validator, if any.
+   *
+   * @return The alias key string, or {@code null} if aliases are not supported.
+   */
+  @Nullable
+  default String getAlias() {
+    return null;
+  }
+}

dynamic-control/src/main/java/io/opentelemetry/contrib/dynamic/policy/TraceSamplingValidator.java

@@ -0,0 +1,75 @@
+/*
+ * Copyright The OpenTelemetry Authors
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+package io.opentelemetry.contrib.dynamic.policy;
+
+import com.fasterxml.jackson.core.JsonProcessingException;
+import com.fasterxml.jackson.databind.JsonNode;
+import com.fasterxml.jackson.databind.ObjectMapper;
+import java.util.logging.Logger;
+import javax.annotation.Nullable;
+
+/**
+ * Validator for trace sampling policies.
+ *
+ * <p>This validator handles the "trace-sampling" policy type and supports the
+ * "trace-sampling.probability" alias.
+ */
+public final class TraceSamplingValidator implements PolicyValidator {
+  private static final Logger logger = Logger.getLogger(TraceSamplingValidator.class.getName());
+  private static final ObjectMapper MAPPER = new ObjectMapper();
+
+  @Override
+  public String getPolicyType() {
+    return "trace-sampling";
+  }
+
+  @Override
+  public String getAlias() {
+    return "trace-sampling.probability";
+  }
+
+  @Override
+  @Nullable
+  public TelemetryPolicy validate(String json) {
+    try {
+      JsonNode node = MAPPER.readTree(json);
+      if (node.has(getPolicyType())) {
+        JsonNode spec = node.get(getPolicyType());
+        if (spec.has("probability")) {
+          JsonNode probNode = spec.get("probability");
+          if (probNode.isNumber()) {
+            double d = probNode.asDouble();
+            if (d >= 0.0 && d <= 1.0) {
+              return new TelemetryPolicy(getPolicyType(), spec);
+            }
+          }
+        }
+      }
+    } catch (JsonProcessingException e) {
+      // Not valid JSON for this validator
+    }
+    logger.info("Invalid trace-sampling JSON: " + json);
+    return null;
+  }
+
+  @Override
+  @Nullable
+  public TelemetryPolicy validateAlias(String key, String value) {
+    if (getAlias() != null && getAlias().equals(key)) {
+      try {
+        double d = Double.parseDouble(value);
+        if (d >= 0.0 && d <= 1.0) {
+          JsonNode spec = MAPPER.createObjectNode().put("probability", d);
+          return new TelemetryPolicy(getPolicyType(), spec);
+        }
+      } catch (NumberFormatException e) {
+        // invalid
+      }
+      logger.info("Ignoring invalid trace-sampling.probability value: " + value);
+    }
+    return null;
+  }
+}