Add service tag (#287)

Signed-off-by: Pavol Loffay <ploffay@redhat.com>

opentracing-api/src/main/java/io/opentracing/tag/Tags.java

@@ -45,6 +45,17 @@ private Tags() {
      */
     public static final String SPAN_KIND_CONSUMER = "consumer";
 
+    /**
+     * The service name for a span, which overrides any default "service name" property defined
+     * in a tracer's config. This tag is meant to only be used when a tracer is reporting spans
+     * on behalf of another service (for example, a service mesh reporting on behalf of the services
+     * it is proxying). This tag does not need to be used when reporting spans for the service the
+     * tracer is running in.
+     *
+     * @see #PEER_SERVICE
+     */
+    public static final StringTag SERVICE = new StringTag("service");
+
     /**
      * HTTP_URL records the url of the incoming request.
      */
@@ -71,7 +82,9 @@ private Tags() {
     public static final StringTag PEER_HOST_IPV6 = new StringTag("peer.ipv6");
 
     /**
-     * PEER_SERVICE records the service name of the peer.
+     * PEER_SERVICE records the service name of the peer service.
+     *
+     * @see #SERVICE
      */
     public static final StringTag PEER_SERVICE = new StringTag("peer.service");