Fix zipkin span serialization with empty attributes (#102)

* ZipkinExporter: change test to verify that tags always serialize as a JSON object

* ZipkinExporter: ensure empty attributes get serialized as a JSON object

Author: hectorj

src/Trace/Exporter/ZipkinExporter.php

@@ -140,6 +140,12 @@ public function convertSpans(TracerInterface $tracer, $headers = null)
                 ? str_pad($span->parentSpanId(), 16, '0', STR_PAD_LEFT)
                 : null;
 
+            $attributes = $span->attributes();
+            if (empty($attributes)) {
+                // force json_encode to render an empty object ("{}") instead of an empty array ("[]")
+                $attributes = new \stdClass();
+            }
+
             $zipkinSpan = [
                 'traceId' => $traceId,
                 'name' => $span->name(),
@@ -150,7 +156,7 @@ public function convertSpans(TracerInterface $tracer, $headers = null)
                 'debug' => $isDebug,
                 'shared' => $isShared,
                 'localEndpoint' => $localEndpoint,
-                'tags' => $span->attributes()
+                'tags' => $attributes
             ];
 
             $zipkinSpans[] = $zipkinSpan;

tests/unit/Trace/Exporter/ZipkinExporterTest.php

@@ -66,7 +66,10 @@ public function testFormatsTrace()
             $this->assertInternalType('int', $span['timestamp']);
             $this->assertInternalType('int', $span['duration']);
 
-            $this->assertInternalType('array', $span['tags']);
+            // make sure we have a JSON object, even when there is no tags
+            $this->assertStringStartsWith('{', \json_encode($span['tags']));
+            $this->assertStringEndsWith('}', \json_encode($span['tags']));
+
             foreach ($span['tags'] as $key => $value) {
                 $this->assertInternalType('string', $key);
                 $this->assertInternalType('string', $value);