Set Coordination component for export partition completion handler

The previous commit covered the four `BackgroundSchedulePool` /
query-handler entry points that talk to ZooKeeper from the partition-
export feature, but missed the completion path. The part-export
completion callback is invoked from a `MergeTreeBackgroundExecutor`
thread, which does not inherit any thread-local component set by the
scheduling task. As a result, when
`enforce_keeper_component_tracking` is on, every Keeper request issued
from `ExportPartitionTaskScheduler::handlePartExportCompletion` (and
its callees `handlePartExportSuccess`,
`handlePartExportFailure`, `tryToMovePartToProcessed`,
`areAllPartsProcessed`) raises `LOGICAL_ERROR: Current component is
empty`, aborting the server in debug builds.

Observed on PR #1718 (commit `e239d42`):
- Stateless tests (amd_debug, sequential)
- Stateless tests (amd_asan, db disk, distributed plan, sequential)
- Stateless tests (amd_debug, distributed plan, s3 storage, sequential)
- Stateless tests (arm_binary, sequential)
- Stateless tests (arm_asan, azure, sequential)
- Many `test_export_replicated_mt_partition_to_*` integration tests

All show the same fatal stack ending in `tryToMovePartToProcessed` ->
`zkutil::ZooKeeper::tryGet`.

Wrap `handlePartExportCompletion` with `Coordination::setCurrentComponent`
to cover all four sub-paths in one place. Also wrap
`ExportPartFromPartitionExportTask::executeStep`, which is the same
threading model (a `MergeTreeBackgroundExecutor` task issuing
`zk->tryCreate`) and would fail identically when
`export_merge_tree_partition_lock_inside_the_task` is enabled.

CI: #1718

Author: zvonand

src/Storages/MergeTree/ExportPartFromPartitionExportTask.cpp

@@ -1,5 +1,6 @@
 #include <Storages/MergeTree/ExportPartFromPartitionExportTask.h>
 #include <Common/ProfileEvents.h>
+#include <Common/ZooKeeper/ZooKeeperCommon.h>
 
 namespace ProfileEvents
 {
@@ -23,6 +24,9 @@ ExportPartFromPartitionExportTask::ExportPartFromPartitionExportTask(
 
 bool ExportPartFromPartitionExportTask::executeStep()
 {
+    /// Runs on a MergeTreeBackgroundExecutor thread, so it does not inherit any component set by the scheduling task.
+    auto component_guard = Coordination::setCurrentComponent("ExportPartFromPartitionExportTask::executeStep");
+
     const auto zk = storage.getZooKeeper();
     const auto part_name = manifest.data_part->name;
 

src/Storages/MergeTree/ExportPartitionTaskScheduler.cpp

@@ -4,6 +4,7 @@
 #include <Interpreters/DatabaseCatalog.h>
 #include <Common/Exception.h>
 #include <Common/ZooKeeper/Types.h>
+#include <Common/ZooKeeper/ZooKeeperCommon.h>
 #include <Common/ProfileEvents.h>
 #include "Storages/MergeTree/ExportPartitionUtils.h"
 #include "Storages/MergeTree/MergeTreePartExportManifest.h"
@@ -271,6 +272,9 @@ void ExportPartitionTaskScheduler::handlePartExportCompletion(
     const StoragePtr & destination_storage,
     const MergeTreePartExportManifest::CompletionCallbackResult & result)
 {
+    /// Invoked from MergeTreeBackgroundExecutor threads, so the component is not inherited from selectPartsToExport.
+    auto component_guard = Coordination::setCurrentComponent("ExportPartitionTaskScheduler::handlePartExportCompletion");
+
     const auto export_path = fs::path(storage.zookeeper_path) / "exports" / export_key;
     const auto processing_parts_path = export_path / "processing";
     const auto processed_part_path = export_path / "processed" / part_name;