encode b3 headers without padding when not 32 chars

Author: castengo

src/oc_propagation_http_b3.erl

@@ -39,7 +39,7 @@ to_headers(#span_ctx{trace_id=TraceId,
                      trace_options=TraceOptions}) ->
     Options = case TraceOptions band 1 of 1 -> "1"; _ -> "0" end,
     %% iolist_to_binary only needed for versions before otp-21
-    EncodedTraceId = iolist_to_binary(io_lib:format("~32.16.0b", [TraceId])),
+    EncodedTraceId = iolist_to_binary(io_lib:format("~.16b", [TraceId])),
     EncodedSpanId = iolist_to_binary(io_lib:format("~16.16.0b", [SpanId])),
     [{?B3_TRACE_ID, EncodedTraceId},
      {?B3_SPAN_ID, EncodedSpanId},

test/oc_span_ctx_SUITE.erl

@@ -134,6 +134,15 @@ b3_encode_decode_headers(_Config) ->
     compare_b3_headers(Encoded, Headers),
     ?assertEqual(Decoded, oc_propagation_http_b3:from_headers(Encoded)),
 
+    %% TraceId: 4bf92f3577b34da6
+    %% SpanId: 00f067aa0ba902b7
+    %% Enabled: true
+    ShortTraceIDHeaders = ?B3_HEADERS(<<"4bf92f3577b34da6">>, <<"00f067aa0ba902b7">>, <<"1">>),
+    ShortTraceIDDecoded = oc_propagation_http_b3:from_headers(ShortTraceIDHeaders),
+    ShortTraceIDEncoded = oc_propagation_http_b3:to_headers(ShortTraceIDDecoded),
+    compare_b3_headers(ShortTraceIDEncoded, ShortTraceIDHeaders),
+    ?assertEqual(ShortTraceIDDecoded, oc_propagation_http_b3:from_headers(ShortTraceIDEncoded)),
+
     %% TraceId: 4bf92f3577b34da6a3ce929d0e0e4736
     %% SpanId: 00f067aa0ba902b7
     %% Enabled: false