Add set_current_span and monitor functions to Tracing

`Tracing.Monitor.monitor(Tracing.current_span())` is a bit on the long
side for something that should 99% of the times do the same thing. We
can add an extra function on `Tracing` and use the current span as
fallback if nothing is given.

lib/tracing.ex

@@ -112,6 +112,11 @@ defmodule Tracing do
     |> :otel_propagator_text_map.extract()
   end
 
+  @doc """
+  Helper function to start Tracing.Monitor
+  """
+  def monitor(span \\ nil), do: Tracing.Monitor.monitor(span)
+
   @doc """
   Use this function when defining an anonymous function that will be used in _another process_.
   This function will ensure that the span in the parent process is the parent span of the span in the spawned process.
@@ -154,7 +159,7 @@ defmodule Tracing do
       parent_span = OpenTelemetry.Tracer.current_span_ctx()
 
       if unquote(opts)[:monitor] do
-        Tracing.Monitor.monitor(parent_span)
+        Tracing.monitor(parent_span)
       end
 
       fn unquote_splicing(fun_args) ->

lib/tracing/monitor.ex

@@ -46,11 +46,16 @@ defmodule Tracing.Monitor do
     {:noreply, state}
   end
 
-  def monitor(span_ctx) do
+  @doc """
+  Start monitoring the given span. If the span is not set, it defaults to `Tracing.current_span()`
+  """
+  def monitor(span \\ nil) do
+    span = span || Tracing.current_span()
+
     if Application.fetch_env!(:opentelemetry, :processors) != [] do
       # monitor first, because the monitor is necessary to clean the ets table.
       :ok = GenServer.call(__MODULE__, {:monitor, self()})
-      true = :ets.insert(__MODULE__, {self(), span_ctx})
+      true = :ets.insert(__MODULE__, {self(), span})
     end
   end
 end