fix(layer): metric splitting, span rename (#5902)

Per [feedback], split the Layer metrics, also finally account for lost
and [re-submitted feedback] on `layer_gc` by renaming it to
`layer_delete`, `Layer::garbage_collect_on_drop` renamed to
`Layer::delete_on_drop`. References to "gc" dropped from metric names
and elsewhere.

Also fixes how the cancellations were tracked: there was one rare
counter. Now there is a top level metric for cancelled inits, and the
rare "download failed but failed to communicate" counter is kept.

Fixes: #6027

[feedback]: #5809 (review)
[re-submitted feedback]: #5108 (comment)

pageserver/src/metrics.rs

@@ -2094,6 +2094,8 @@ pub fn preinitialize_metrics() {
     // Tenant manager stats
     Lazy::force(&TENANT_MANAGER);
 
+    Lazy::force(&crate::tenant::storage_layer::layer::LAYER_IMPL_METRICS);
+
     // countervecs
     [&BACKGROUND_LOOP_PERIOD_OVERRUN_COUNT]
         .into_iter()

pageserver/src/tenant/storage_layer.rs

@@ -4,7 +4,7 @@ pub mod delta_layer;
 mod filename;
 pub mod image_layer;
 mod inmemory_layer;
-mod layer;
+pub(crate) mod layer;
 mod layer_desc;
 
 use crate::context::{AccessStatsBehavior, RequestContext};