add example that shows accessing request headers

[davepacheco]

Someone asked me about how to access request headers from a dropshot handler, so I threw together an example that shows this.

[davepacheco]

I ran this example like this:

$ cargo run --example=request-headers
...
Jun 13 21:59:14.178 INFO listening, local_addr: 127.0.0.1:56495

Here are some example requests using it. First, without specifying the demo header:

$ curl -i 127.0.0.1:56495/header-example-generic
HTTP/1.1 200 OK
content-type: application/json
x-request-id: 7b5f1329-c01a-44a8-8e51-4766621700ff
content-length: 24
date: Mon, 13 Jun 2022 21:59:27 GMT

"value for header: None"

Now, specifying a value:

$ curl -H 'demo-header: demo-value' -i 127.0.0.1:56495/header-example-generic
HTTP/1.1 200 OK
content-type: application/json
x-request-id: fcbd1a7f-0978-492b-b9c5-9fe8b4c267fb
content-length: 40
date: Mon, 13 Jun 2022 21:59:49 GMT

"value for header: Some(\"demo-value\")"

If you specify two values, you only get one of them:

$ curl -H 'demo-header: demo-value-1' -H 'demo-header: demo-value-2' -i 127.0.0.1:56495/header-example-generic
HTTP/1.1 200 OK
content-type: application/json
x-request-id: ba14f906-d9ff-4b7d-8749-17836823971d
content-length: 42
date: Mon, 13 Jun 2022 22:07:22 GMT

"value for header: Some(\"demo-value-1\")"

I confirmed with curl -v that curl is sending both, but the code in the example uses http::HeaderMap::get, which only returns the first value. There's a comment in the example about this and pointing people at the docs for getting other values out.

[ahl]

let's get that money

[StaringSkyward]

This is exactly what I needed to figure out JWT auth with dropshot. Thanks @davepacheco!