Add a create_http_headers_for_my_span() zipkin_span method

The rationale is that I wanted to create a Kind=CLIENT span in server A
with a new span_id (and parent_span_id of the span that had been active
at the time), then generate headers with that same new span_id for the
request to server B, where the span there would have `shared=True` in
its local-root-span.

So that gives me 2 spans total, the parent span, and a single shared
span with cs/cr happening on server A and sr/ss happening on server B.

If I'd just used create_http_headers_for_new_span(), then I'd have had
three total spans: the parent span, a weird span with only cs/cr from
server A, and a 3rd span with only sr/ss from server B.

I'm not sure if not having this caused some of the confusion in
Issue #126 or not...

py_zipkin/zipkin.py

@@ -617,6 +617,29 @@ def override_span_name(self, name):
         if self.logging_context:
             self.logging_context.span_name = name
 
+    def create_http_headers_for_my_span(self):
+        """
+        Generate the headers for sharing this context object's zipkin_attrs
+        with a shared span on another host.
+
+        If this instance doesn't have zipkin_attrs set, for some reason, an
+        empty dict is returned.
+
+        :returns: dict containing (X-B3-TraceId, X-B3-SpanId, X-B3-ParentSpanId,
+                    X-B3-Flags and X-B3-Sampled) or an empty dict.
+        """
+        zipkin_attrs = self.zipkin_attrs
+        if not zipkin_attrs:
+            return {}
+
+        return {
+            'X-B3-TraceId': zipkin_attrs.trace_id,
+            'X-B3-SpanId': zipkin_attrs.span_id,
+            'X-B3-ParentSpanId': zipkin_attrs.parent_span_id,
+            'X-B3-Flags': '0',
+            'X-B3-Sampled': '1' if zipkin_attrs.is_sampled else '0',
+        }
+
 
 def _validate_args(kwargs):
     if 'kind' in kwargs:

tests/zipkin_test.py

@@ -844,6 +844,48 @@ def test_override_span_name(self):
             assert span.span_name == 'new_name'
             assert span.logging_context.span_name == 'new_name'
 
+    def test_create_http_headers_for_my_span_no_zipkin_attrs(self):
+        with zipkin.zipkin_client_span(
+                service_name='test_service',
+                span_name='test_span',
+                transport_handler=MockTransportHandler(),
+                sample_rate=100.0,
+        ) as span:
+            # Not sure how this could ever happen in real life, but if it
+            # did...
+            span.zipkin_attrs = None
+            assert {} == span.create_http_headers_for_my_span()
+
+    def test_create_http_headers_for_my_span_is_sampled(self):
+        with zipkin.zipkin_client_span(
+                service_name='test_service',
+                span_name='test_span',
+                transport_handler=MockTransportHandler(),
+                sample_rate=100.0,
+        ) as span:
+            assert {
+                'X-B3-TraceId': span.zipkin_attrs.trace_id,
+                'X-B3-SpanId': span.zipkin_attrs.span_id,
+                'X-B3-ParentSpanId': span.zipkin_attrs.parent_span_id,
+                'X-B3-Flags': '0',
+                'X-B3-Sampled': '1',
+            } == span.create_http_headers_for_my_span()
+
+    def test_create_http_headers_for_my_span_is_NOT_sampled(self):
+        with zipkin.zipkin_client_span(
+                service_name='test_service',
+                span_name='test_span',
+                transport_handler=MockTransportHandler(),
+                sample_rate=0.0,
+        ) as span:
+            assert {
+                'X-B3-TraceId': span.zipkin_attrs.trace_id,
+                'X-B3-SpanId': span.zipkin_attrs.span_id,
+                'X-B3-ParentSpanId': span.zipkin_attrs.parent_span_id,
+                'X-B3-Flags': '0',
+                'X-B3-Sampled': '0',
+            } == span.create_http_headers_for_my_span()
+
 
 def test_zipkin_client_span():
     context = zipkin.zipkin_client_span('test_service', 'test_span')