Baggage member values are not being decoded when extracted from carriers

[williamgcampbell]

Description

According to the spec, Baggage header values should be encoded in transit.

The Baggage propagator appears to be encoding Baggage Member values on Inject but never decoding them on Extract. For multiple passive hops, the Baggage Member value will continue getting encoded.

Environment

• OS: linux/mac
• Architecture: x86_64
• Go Version: 1.17
• opentelemetry-go version: v1.2.0

Steps To Reproduce

1. Set a baggage header with a member value that requires encoding
2. Pass the baggage through multiple Inject/Extracts of the Baggage propagator.

Here's a test to demonstrate the behavior in isolation:

func TestBaggage(t *testing.T) {
	propagator := propagation.Baggage{}
	hc := propagation.HeaderCarrier{}
	hc.Set("baggage", "myKey=1%3D1")

	want := "1%3D1"
        ctx := context.Background()

	ctx = propagator.Extract(ctx, hc)
	actual := baggage.FromContext(ctx).Member("myKey").Value()
	require.Equal(t, want, actual)

        // inject will encode the '%' literal from the already encoded value
	propagator.Inject(ctx, hc)

	ctx = propagator.Extract(ctx, hc)
	actual = baggage.FromContext(ctx).Member("myKey").Value()
	require.Equal(t, want, actual) // fail
}

Expected behavior

The Baggage Member value should be decoded when being extracted onto the Context.