Monitoring: trace history, trace search, and x:Trace over the stored traces (MON-10 to MON-17, MON-32)

A lossy collector subscriber keeps each inbound SMTP session that reached
MAIL FROM and each delivery attempt, info level and above and never raw
I/O, at most 1,000 events with strings cut at 4 KiB, and writes it when
the span closes as an x:Trace under the telemetry key class, scheduling
its indexing. The index task builds a document of event types, queue ids
and keywords when indexTelemetry is on. x:Trace/get derives timestamp,
from, to and size; /query filters by opening event, text, queueId and
time; /set destroys only. The data purge honours holdTracesFor. The shared
tracing and webhook suites run.

crates/services/src/task_manager/maintenance.rs

@@ -248,6 +248,18 @@ async fn store_maintenance(
             {
                 trc::error!(err.details("Failed to purge metric history"));
             }
+            if let Some(keep) = retention.hold_traces_for
+                && !server.tracing_store().is_none()
+            {
+                use common::telemetry::tracers::store::TracingStore;
+                if let Err(err) = server
+                    .tracing_store()
+                    .purge_spans(keep.into_inner(), Some(server.search_store()))
+                    .await
+                {
+                    trc::error!(err.details("Failed to purge trace history"));
+                }
+            }
 
             trc::event!(
                 Store(StoreEvent::DataStorePurged),