Catch Cancelled and don't print a warning for it (#9121)

In the `imitate_synthetic_size_calculation_worker` function, we might
obtain the `Cancelled` error variant instead of hitting the cancellation
token based path. Therefore, catch `Cancelled` and handle it analogously
to the cancellation case.
 
Fixes #8886.

pageserver/src/tenant/timeline/eviction_task.rs

@@ -30,8 +30,8 @@ use crate::{
     pgdatadir_mapping::CollectKeySpaceError,
     task_mgr::{self, TaskKind, BACKGROUND_RUNTIME},
     tenant::{
-        storage_layer::LayerVisibilityHint, tasks::BackgroundLoopKind, timeline::EvictionError,
-        LogicalSizeCalculationCause, Tenant,
+        size::CalculateSyntheticSizeError, storage_layer::LayerVisibilityHint,
+        tasks::BackgroundLoopKind, timeline::EvictionError, LogicalSizeCalculationCause, Tenant,
     },
 };
 
@@ -557,6 +557,8 @@ impl Timeline {
             gather_result = gather => {
                 match gather_result {
                     Ok(_) => {},
+                    // It can happen sometimes that we hit this instead of the cancellation token firing above
+                    Err(CalculateSyntheticSizeError::Cancelled) => {}
                     Err(e) => {
                         // We don't care about the result, but, if it failed, we should log it,
                         // since consumption metric might be hitting the cached value and