Asgi request headers are not retrieved properly

[garnertan]

Environment:
Python: python3.8

fastapi==0.63.0
opentelemetry-api==0.16b1
opentelemetry-sdk==0.16b1
opentelemetry-instrumentation-fastapi==0.16b1
opentelemetry-exporter-google-cloud==0.16b1
opentelemetry-tools-google-cloud==0.16b1

When using CloudTraceFormatPropagator for GCP, X-Cloud-Trace-Context header cannot be retrieved.

Steps to reproduce
Describe exactly how to reproduce the error. Include a code sample if applicable.

# server.py
import uvicorn
from fastapi import FastAPI, Request
from opentelemetry import trace
from opentelemetry.propagators import set_global_textmap
from opentelemetry.instrumentation.fastapi import FastAPIInstrumentor
from opentelemetry.sdk.trace import TracerProvider
from opentelemetry.tools import cloud_trace_propagator

set_global_textmap(cloud_trace_propagator.CloudTraceFormatPropagator())

app = FastAPI()

tracer = trace.get_tracer("test")
FastAPIInstrumentor.instrument_app(app)

@app.get("/trace")
async def test(r: Request):
    with tracer.start_as_current_span("test") as span:
        trace_id = span.get_span_context().trace_id
        print(f"{trace_id:32x}")  # should print trace ID from `X-Cloud-Trace-Context` header value

uvicorn.run(app)

# client.py
import requests

r = requests.Session()
r.headers.setdefault("X-Cloud-Trace-Context",
                        "f3ef5c2ede256aa77491057e600eca11/15104302039794794507;o=1")
r.get("http://localhost:8000/trace")

What is the expected behavior?
Printed value should be f3ef5c2ede256aa77491057e600eca11 based from the header sent

What is the actual behavior?
A newly generated value everything /trace is called

Additional context
X-Cloud-Trace-Context header value is not retrieved properly in CloudTraceFormatPropagator