Span metadata must be string values.

dd-trace/src/main/java/datadog/trace/tracer/Span.java

@@ -99,7 +99,7 @@ public interface Span {
   /**
    * Get all the metadata attached to this span.
    *
-   * @return
+   * @return immutable map of span metadata.
    */
   Map<String, Object> getMeta();
 

dd-trace/src/main/java/datadog/trace/tracer/SpanImpl.java

@@ -4,6 +4,7 @@ import com.fasterxml.jackson.annotation.JsonAutoDetect;
 import com.fasterxml.jackson.annotation.JsonAutoDetect.Visibility;
 import com.fasterxml.jackson.annotation.JsonFormat;
 import com.fasterxml.jackson.annotation.JsonGetter;
+import com.fasterxml.jackson.annotation.JsonIgnore;
 import com.fasterxml.jackson.core.JsonGenerator;
 import com.fasterxml.jackson.databind.SerializerProvider;
 import com.fasterxml.jackson.databind.annotation.JsonSerialize;
@@ -13,6 +14,7 @@ import java.io.IOException;
 import java.io.PrintWriter;
 import java.io.StringWriter;
 import java.math.BigInteger;
+import java.util.Collections;
 import java.util.HashMap;
 import java.util.List;
 import java.util.Map;
@@ -213,9 +215,19 @@ class SpanImpl implements Span {
   }
 
   @Override
-  @JsonGetter("meta")
+  @JsonIgnore
   public synchronized Map<String, Object> getMeta() {
-    final Map<String, Object> result = new HashMap<>(meta.size());
+    return Collections.unmodifiableMap(meta);
+  }
+
+  /**
+   * The agent expects meta's values to be strings.
+   *
+   * @return a copy of meta with all values converted to strings.
+   */
+  @JsonGetter("meta")
+  synchronized Map<String, String> getMetaString() {
+    final Map<String, String> result = new HashMap<>(meta.size());
     for (final Map.Entry<String, Object> entry : meta.entrySet()) {
       result.put(entry.getKey(), String.valueOf(entry.getValue()));
     }

dd-trace/src/test/groovy/datadog/trace/tracer/JsonSpan.groovy

@@ -43,7 +43,7 @@ class JsonSpan {
   @JsonCreator
   JsonSpan() {}
 
-  JsonSpan(Span span) {
+  JsonSpan(SpanImpl span) {
     traceId = new BigInteger(span.getContext().getTraceId())
     parentId = new BigInteger(span.getContext().getParentId())
     spanId = new BigInteger(span.getContext().getSpanId())
@@ -58,6 +58,6 @@ class JsonSpan {
 
     error = span.isErrored() ? 1 : 0
 
-    meta = span.getMeta()
+    meta = span.getMetaString()
   }
 }

dd-trace/src/test/groovy/datadog/trace/tracer/SpanImplTest.groovy

@@ -129,7 +129,8 @@ class SpanImplTest extends Specification {
     span.setMeta("boolean.key", true)
 
     then:
-    span.getMeta() == ["number.key": "123", "string.key": "meta string", "boolean.key": "true"]
+    span.getMeta() == ["number.key": 123, "string.key": "meta string", "boolean.key": true]
+    span.getMetaString() == ["number.key": "123", "string.key": "meta string", "boolean.key": "true"]
   }
 
   def "test meta setter on finished span for #key"() {
@@ -227,7 +228,7 @@ class SpanImplTest extends Specification {
 
     when: "finish/finalize span"
     span."$method"(*methodArgs)
-    
+
     then: "interceptors called"
     interceptors.reverseEach({ interceptor ->
       then: