Document baggage propagation

Thanks for this great project! However, I noticed propagating baggage across network calls is not documented. I needed to go through the code to find a solution.

Here is a example code snippet we could clean up and add to the cookbook:
https://opentelemetry.io/docs/concepts/signals/traces/#context-propagation

Below, api1 calls api2:

api1:

from flask import Flask
import requests
from opentelemetry import trace, propagators, baggage
from opentelemetry.instrumentation.flask import FlaskInstrumentor
from opentelemetry.instrumentation.requests import RequestsInstrumentor
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
from opentelemetry.exporter.jaeger.thrift import JaegerExporter

app = Flask(__name__)

trace.set_tracer_provider(TracerProvider())
trace.get_tracer_provider().add_span_processor(BatchSpanProcessor(ConsoleSpanExporter()))

jaeger_exporter = JaegerExporter(
    agent_host_name="localhost",  # Set your Jaeger agent host
    agent_port=6831,  # Set your Jaeger agent port
)
trace.get_tracer_provider().add_span_processor(BatchSpanProcessor(jaeger_exporter))

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)

api2:

from flask import Flask, request
from opentelemetry import trace, baggage
from opentelemetry.instrumentation.flask import FlaskInstrumentor
from opentelemetry.trace.propagation.tracecontext import TraceContextTextMapPropagator
from opentelemetry.sdk.trace import TracerProvider
from opentelemetry.sdk.trace.export import ConsoleSpanExporter, BatchSpanProcessor
from opentelemetry.exporter.jaeger.thrift import JaegerExporter
import time
from opentelemetry.baggage.propagation import W3CBaggagePropagator

app = Flask(__name__)

trace.set_tracer_provider(TracerProvider())
trace.get_tracer_provider().add_span_processor(BatchSpanProcessor(ConsoleSpanExporter()))

jaeger_exporter = JaegerExporter(
    agent_host_name="localhost",  # Set your Jaeger agent host
    agent_port=6831,  # Set your Jaeger agent port
)
trace.get_tracer_provider().add_span_processor(BatchSpanProcessor(jaeger_exporter))

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)

Start with the linked context propagation documentation and the Python example in this issue, then locate the repository's cookbook structure. Add a cleaned example showing baggage propagation between api1 and api2; done when the cookbook explains the flow clearly and the example is suitable for readers to follow.