Previously, stacks were built with `Layer::and_then`. This pattern
severely impacts compile-times as stack complexity grows.
In order to ameliorate this, `app::main` has been changed to build
stacks from the "bottom" (endpoint client) to "top" (serverside
connection) by _push_-ing Layers onto a concrete stack, i.e. and not
composing layers for an abstract stack.
While doing this, we take the oppportunity to remove a ton of
now-unnecessary `PhantomData`. A new, dedicated `phantom_data` stack
module can be used to aid type inference as needed.
Other stack utilities like `map_target` and `map_err` have been
introduced to assist this transition.
Furthermore, all instances of `Layer::new` have been changed to a free
`fn layer` to improve readability.
This change sets up two upcoming changes: a stack-oriented `controller`
client and, subsequently, service-profile-based routing.
* Prepare HTTP metrics for per-route classification
In order to support Service Profiles, the proxy will add a new scope of
HTTP metrics prefixed with `route_`, i.e. so that the proxy exposes
`request_total` and `route_request_total` independently.
Furthermore, the proxy must be able to use different
response-classification logic for each route, and this classification
logic should apply to both metrics scopes.
This alters the `proxy::http::metrics` module so that:
1. HTTP metrics may be scoped with a prefix (as the stack is described).
2. The HTTP metrics layer now discovers the classifier by trying to
extract it from each request's extensions or fall back to a `Default`
implementation. Only a default implementation is used presently.
3. It was too easy to use the `Classify` trait API incorrectly.
Non-default classify implementation could cause a runtime panic!
The API has been changed so that the type system ensures correct
usage.
4. The HTTP classifier must be configurable per-request. In order to do
this, we expect a higher stack layer will add response classifiers to
request extensions when appropriate (i.e., in a follow-up).
Finally, the `telemetry::Report` type requires updating every time a new
set of metrics is added. We don't need a struct to represent this.
`FmtMetrics::and_then` has been added as a combinator so that a fixed
type is not necessary.
* Fix linkerd2-metrics test compilation
The `quickcheck` dependency was lost when the subcrate was split out.
This change restores the dependency.
Fixeslinkerd/linkerd2#1685
* Run tests for all packages in `make test`
As we extract subcrates from the `src/` directory, the repository root
becomes a bit cluttered. This change moves these subcrates into a `lib`
directory.
Oliver Gould