Creating a sample for the header propagation package and some various
misc UX improvements based on building the sample.
A small list:
- allow duplicate inbound names
- de-dupe based on outbound names
- add sugar for configuration
- simplify pattern for transforming values
- add error message for missing middleware
Also a few small perf things.
I started this out by wanting to remove the following from the
configuration pattern:
```C#
options.Headers.Add("X-TraceId", null);
```
This pattern with null is undiscoverable, but we didn't provide
something simpler. The most common case was to add a custom collection
type so we can define sugar methods.
The next realization is that in practical case (dist tracing sample) you
either way to *key* off of the same inbound header twice, or you don't
have an inbound header at all, and you will synthesize the value every
time. This means that the way we're treating inbound header names is a
bit wrong. We don't want inbound header names to be unique, we want
*outbound header names to be unique*.
Next, I want to consolidate DefaultValue and ValueFactory. The problems
I saw with this:
- DefaultValue is a trap. It's rare to use a static value.
- ValueFactory really wants the header name *and* value
I think what's there now is much more terse to work with.
Ryan Nowak