docs(app): Document route metrics' label set types

this adds some documentation, mostly pointing out that this submodule
exists to provide trait implementations to encode label sets, for use in
accessing metrics from a `Family`.

Signed-off-by: katelyn martin <[email protected]>

linkerd/app/outbound/src/http/logical/policy/route/metrics/labels.rs

@@ -1,30 +1,44 @@
 //! Prometheus label types.
+//!
+//! This submodule contains types that implement [`EncodeLabelSet`], [`EncodeLabelSetMut`], and
+//! [`EncodeLabelValue`]. These may be used to work with a labeled
+//! [`Family`][prometheus_client::metrics::family::Family] of metrics.
+//!
+//! Use [`Family::get_or_create()`][prometheus_client::metrics::family::Family::get_or_create]
+//! to retrieve, or create should it not exist, a metric with a given set of label values.
+
 use linkerd_app_core::{errors, metrics::prom::EncodeLabelSetMut, proxy::http, Error as BoxError};
 use prometheus_client::encoding::*;
 
 use crate::{BackendRef, ParentRef, RouteRef};
 
+/// Prometheus labels for a route resource, usually a service.
 #[derive(Clone, Debug, Hash, PartialEq, Eq)]
 pub struct Route(pub ParentRef, pub RouteRef);
 
+/// Prometheus labels for a backend resource.
 #[derive(Clone, Debug, Hash, PartialEq, Eq)]
 pub struct RouteBackend(pub ParentRef, pub RouteRef, pub BackendRef);
 
+/// Prometheus labels for a route's response.
 #[derive(Clone, Debug, Hash, PartialEq, Eq)]
 pub struct Rsp<P, L>(pub P, pub L);
 
+/// Prometheus labels for an HTTP response.
 #[derive(Clone, Debug, Hash, PartialEq, Eq)]
 pub struct HttpRsp {
     pub status: Option<http::StatusCode>,
     pub error: Option<Error>,
 }
 
+/// Prometheus labels for a gRPC response.
 #[derive(Clone, Debug, Hash, PartialEq, Eq)]
 pub struct GrpcRsp {
     pub status: Option<tonic::Code>,
     pub error: Option<Error>,
 }
 
+/// Prometheus labels representing an error.
 #[derive(Copy, Clone, Debug, Hash, PartialEq, Eq)]
 pub enum Error {
     FailFast,
@@ -170,6 +184,7 @@ impl EncodeLabelSet for GrpcRsp {
 // === impl Error ===
 
 impl Error {
+    /// Returns an [`Error`] or a status code, given a boxed error.
     pub fn new_or_status(error: &BoxError) -> Result<Self, u16> {
         use super::super::super::errors as policy;
         use crate::http::h2::{H2Error, Reason};