Add b3multi option to OTEL_PROPAGATORS

According to [OTEL Spec][1] there are `b3` and `b3multi`. The latter is
supposed to be used when [Multi B3 format][2] is used. This is exactly
what b3_propagators() currently is doing.

[1]: https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/sdk-environment-variables.md#general-sdk-configuration
[2]: https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/context/api-propagators.md#b3-extract

apps/opentelemetry/src/otel_configuration.erl

@@ -174,6 +174,8 @@ transform(propagators, PropagatorsString) when is_list(PropagatorsString) ->
 
 transform(propagator, "tracecontext") ->
     fun otel_tracer_default:w3c_propagators/0;
+transform(propagator, "b3multi") ->
+    fun otel_tracer_default:b3_propagators/0;
 transform(propagator, "baggage") ->
     fun otel_baggage:get_text_map_propagators/0;
 transform(propagator, Propagator) ->

apps/opentelemetry/test/otel_configuration_SUITE.erl

@@ -16,7 +16,8 @@
 all() ->
     [empty_os_environment, sampler, sampler_parent_based, sampler_parent_based_zero,
      sampler_trace_id, sampler_trace_id_default, sampler_parent_based_one,
-     log_level, propagators, otlp_exporter, jaeger_exporter, zipkin_exporter, none_exporter].
+     log_level, propagators, propagators_b3multi, otlp_exporter, jaeger_exporter,
+     zipkin_exporter, none_exporter].
 
 init_per_testcase(empty_os_environment, Config) ->
     Vars = [],
@@ -32,6 +33,12 @@ init_per_testcase(propagators, Config) ->
 
     setup_env(Vars),
 
+    [{os_vars, Vars} | Config];
+init_per_testcase(propagators_b3multi, Config) ->
+    Vars = [{"OTEL_PROPAGATORS", "b3multi"}],
+
+    setup_env(Vars),
+
     [{os_vars, Vars} | Config];
 init_per_testcase(sampler, Config) ->
     Vars = [{"OTEL_TRACES_SAMPLER", "parentbased_always_off"}],
@@ -168,6 +175,13 @@ propagators(_Config) ->
 
     ok.
 
+propagators_b3multi(_Config) ->
+    ?assertIsSubset([{log_level, error},
+                     {propagators, [fun otel_tracer_default:b3_propagators/0]}],
+                    otel_configuration:merge_with_os([{log_level, error}])),
+
+    ok.
+
 otlp_exporter(_Config) ->
     ?assertMatch({traces_exporter, {opentelemetry_exporter, #{}}},
                  lists:keyfind(traces_exporter, 1, otel_configuration:merge_with_os([]))),