sdk: Specify to_json indent from arguments (#718)

We want to get a json of span without indent, but it's not possible with Span#to_json because the method uses hard-coded indent. We currently use a workaround of json.loads(span.to_json()) which is not efficient in the performance.

Co-authored-by: alrex <aboten@lightstep.com>
Co-authored-by: Yusuke Tsutsumi <yusuke@tsutsumi.io>

opentelemetry-sdk/src/opentelemetry/sdk/trace/__init__.py

@@ -360,7 +360,7 @@ def _format_links(links):
             f_links.append(f_link)
         return f_links
 
-    def to_json(self):
+    def to_json(self, indent=4):
         parent_id = None
         if self.parent is not None:
             if isinstance(self.parent, Span):
@@ -397,7 +397,7 @@ def to_json(self):
         f_span["events"] = self._format_events(self.events)
         f_span["links"] = self._format_links(self.links)
 
-        return json.dumps(f_span, indent=4)
+        return json.dumps(f_span, indent=indent)
 
     def get_context(self):
         return self.context