Change trace name fallback to be more informative (#192)

Signed-off-by: Joe Farro <joef@uber.com>

src/constants/index.js

@@ -13,4 +13,4 @@
 // limitations under the License.
 
 export const FALLBACK_DAG_MAX_NUM_SERVICES = 100;
-export const FALLBACK_TRACE_NAME = '<cannot-find-trace-name>';
+export const FALLBACK_TRACE_NAME = '<trace-without-root-span>';