Supply example on Document baggage propagation (open-telemetry#3716)

Co-authored-by: Phillip Carter <[email protected]>

Author: pegasas

content/en/docs/languages/python/propagation.md

@@ -0,0 +1,96 @@
+---
+title: Propagation
+description: Context propagation for the Python SDK
+aliases: [api/propagation]
+weight: 65
+cSpell:ignore: tracestate
+---
+
+Propagation is the mechanism that moves data between services and processes.
+Although not limited to tracing, it is what allows traces to build causal
+information about a system across services that are arbitrarily distributed
+across process and network boundaries.
+
+## Manual W3C Trace Context Propagation
+
+The following generic example demonstrates how you can propagate trace context
+manually.
+
+First, on the sending service, you'll need to inject the current `context`:
+
+```python
+from flask import Flask
+import requests
+from opentelemetry import trace, propagators, baggage
+from opentelemetry.trace.propagation.tracecontext import TraceContextTextMapPropagator
+from opentelemetry.baggage.propagation import W3CBaggagePropagator
+from opentelemetry.sdk.trace import TracerProvider
+from opentelemetry.sdk.trace.export import ConsoleSpanExporter, BatchSpanProcessor
+
+app = Flask(__name__)
+
+trace.set_tracer_provider(TracerProvider())
+trace.get_tracer_provider().add_span_processor(BatchSpanProcessor(ConsoleSpanExporter()))
+
+tracer = trace.get_tracer(__name__)
+
+@app.route('/')
+def hello():
+    with tracer.start_as_current_span("api1_span") as span:
+        ctx = baggage.set_baggage("hello", "world")
+
+        headers = {}
+        W3CBaggagePropagator().inject(headers, ctx)
+        TraceContextTextMapPropagator().inject(headers, ctx)
+        print(headers)
+
+        response = requests.get('http://127.0.0.1:5001/', headers=headers)
+        return f"Hello from API 1! Response from API 2: {response.text}"
+
+if __name__ == '__main__':
+    app.run(port=5002)
+```
+
+On the receiving service, you'll need to extract `context` (for example, from
+parsed HTTP headers) and then set them as the current trace context.
+
+```python
+from flask import Flask, request
+from opentelemetry import trace, baggage
+from opentelemetry.trace.propagation.tracecontext import TraceContextTextMapPropagator
+from opentelemetry.sdk.trace import TracerProvider
+from opentelemetry.sdk.trace.export import ConsoleSpanExporter, BatchSpanProcessor
+from opentelemetry.baggage.propagation import W3CBaggagePropagator
+
+app = Flask(__name__)
+
+trace.set_tracer_provider(TracerProvider())
+trace.get_tracer_provider().add_span_processor(BatchSpanProcessor(ConsoleSpanExporter()))
+
+tracer = trace.get_tracer(__name__)
+
+@app.route('/')
+def hello():
+    # Example: Log headers received in the request in API 2
+    headers = dict(request.headers)
+    print(f"Received headers: {headers}")
+    carrier ={'traceparent': headers['Traceparent']}
+    ctx = TraceContextTextMapPropagator().extract(carrier=carrier)
+    print(f"Received context: {ctx}")
+
+    b2 ={'baggage': headers['Baggage']}
+    ctx2 = W3CBaggagePropagator().extract(b2, context=ctx)
+    print(f"Received context2: {ctx2}")
+
+    # Start a new span
+    with tracer.start_span("api2_span", context=ctx2):
+       # Use propagated context
+        print(baggage.get_baggage('hello', ctx2))
+        return "Hello from API 2!"
+
+if __name__ == '__main__':
+    app.run(port=5001)
+```
+
+From there, when you have a deserialized active context, you can create spans
+that will be a part of the same trace from the other service.