wip: restructure tracer around registry

This is to be split into separate OTP applications to make the
distinction clearer. For now the SDK registry is started when
opentelemetry application is started. The registry is responsible
for tracking the tracers, the sampler and span processors.

The registry also currently sets a ctx and span module to use. I
think this will change when context is split out.  I'd also like
to consider not having the span module be configurable in the
SDK but always use ot_span_ets and if the user needs a fancy form
of span handling they can replace the SDK if they can't get by
with using span processors.

src/opentelemetry.app.src

@@ -8,11 +8,11 @@
     stdlib,
     wts
    ]},
-  {env, [{tracer, {ot_tracer_default, #{span => {ot_span_ets, []},
-                                        ctx => {ot_ctx_pdict, []}}}},
-         {sampler, {always_on, #{}}},
-         {exporter, [{exporters, []},
-                     {scheduled_delay_ms, 30000}]}]},
+  {env, [{sampler, {always_on, #{}}},
+         {processors, [%% #{id => my_processor,
+                       %%   module => ot_batch_processor,
+                       %%   config => #{}}
+                        ]}]},
   {modules, []},
 
   {licenses, ["Apache 2.0"]},

src/opentelemetry.erl

@@ -28,7 +28,10 @@
 %%%-------------------------------------------------------------------------
 -module(opentelemetry).
 
--export([generate_trace_id/0,
+-export([set_default_tracer_registry/1,
+         get_tracer/0,
+         get_tracer/1,
+         generate_trace_id/0,
          generate_span_id/0]).
 
 -include("opentelemetry.hrl").
@@ -93,6 +96,15 @@
 
 -type http_headers()       :: [{unicode:unicode_binary(), unicode:unicode_binary()}].
 
+set_default_tracer_registry(TracerRegistry) ->
+    persistent_term:put({?MODULE, default_tracer_registry}, TracerRegistry).
+
+get_tracer() ->
+    (persistent_term:get({?MODULE, default_tracer_registry}, ot_registry_api)):get().
+
+get_tracer(Name) ->
+    (persistent_term:get({?MODULE, default_tracer_registry}, ot_registry_api)):get(Name).
+
 %%--------------------------------------------------------------------
 %% @doc
 %% Generates a 128 bit random integer to use as a trace id.

src/opentelemetry_app.erl

@@ -23,18 +23,7 @@
 
 start(_StartType, _StartArgs) ->
     Opts = application:get_all_env(opentelemetry),
-
-    {sampler, {Sampler, SamplerOpts}} = lists:keyfind(sampler, 1, Opts),
-    SamplerFun = ot_sampler:setup(Sampler, SamplerOpts),
-
-    %% The default sampler implementation must be passed to the tracer chosen.
-    %% Since the tracer is started after the rest of the supervision tree has started
-    %% and we only get its children, we must instantiate the sampler before anything else.
-    %% The sampler turns out to be instantiated before any supervision tree is started.
-    {tracer, {Tracer, TracerOpts}} = lists:keyfind(tracer, 1, Opts),
-    TracerChildren = ot_tracer:setup(Tracer, TracerOpts, SamplerFun),
-
-    opentelemetry_sup:start_link(TracerChildren, Opts).
+    opentelemetry_sup:start_link(Opts).
 
 stop(_State) ->
     ok.

src/opentelemetry_sup.erl

@@ -19,32 +19,28 @@
 
 -behaviour(supervisor).
 
--export([start_link/2]).
+-export([start_link/1]).
 
 -export([init/1]).
 
 -define(SERVER, ?MODULE).
 
-start_link(Children, Opts) ->
-    supervisor:start_link({local, ?SERVER}, ?MODULE, [Children, Opts]).
+start_link(Opts) ->
+    supervisor:start_link({local, ?SERVER}, ?MODULE, [Opts]).
 
-init([Children, Opts]) ->
+init([Opts]) ->
     SupFlags = #{strategy => one_for_one,
-                 intensity => 0,
-                 period => 1},
-
-    ExporterOpts = proplists:get_value(exporter, Opts, []),
-    Exporter = #{id => ot_exporter,
-                 start => {ot_exporter, start_link, [ExporterOpts]},
-                 restart => permanent,
-                 shutdown => 1000,
-                 type => worker,
-                 modules => [ot_exporter]},
-
-    ChildSpecs = [#{id => ot_span_sup,
-                    start => {ot_span_sup, start_link, [Opts]},
-                    type => supervisor},
-                  Exporter | Children],
+                 intensity => 1,
+                 period => 5},
+
+    RegistrySup = #{id => ot_registry_sup,
+                    start => {ot_registry_sup, start_link, [Opts]},
+                    restart => permanent,
+                    shutdown => 1000,
+                    type => supervisor,
+                    modules => [ot_registry_sup]},
+
+    ChildSpecs = [RegistrySup],
     {ok, {SupFlags, ChildSpecs}}.
 
 %% internal functions