feat: add S3 bucket size and object count metrics (#7776)

* feat: add S3 bucket size and object count metrics

Adds periodic collection of bucket size metrics:
- SeaweedFS_s3_bucket_size_bytes: logical size (deduplicated across replicas)
- SeaweedFS_s3_bucket_physical_size_bytes: physical size (including replicas)
- SeaweedFS_s3_bucket_object_count: object count (deduplicated)

Collection runs every 1 minute via background goroutine that queries
filer Statistics RPC for each bucket's collection.

Also adds Grafana dashboard panels for:
- S3 Bucket Size (logical vs physical)
- S3 Bucket Object Count

* address PR comments: fix bucket size metrics collection

1. Fix collectCollectionInfoFromMaster to use master VolumeList API
   - Now properly queries master for topology info
   - Uses WithMasterClient to get volume list from master
   - Correctly calculates logical vs physical size based on replication

2. Return error when filerClient is nil to trigger fallback
   - Changed from 'return nil, nil' to 'return nil, error'
   - Ensures fallback to filer stats is properly triggered

3. Implement pagination in listBucketNames
   - Added listBucketPageSize constant (1000)
   - Uses StartFromFileName for pagination
   - Continues fetching until fewer entries than limit returned

4. Handle NewReplicaPlacementFromByte error and prevent division by zero
   - Check error return from NewReplicaPlacementFromByte
   - Default to 1 copy if error occurs
   - Add explicit check for copyCount == 0

* simplify bucket size metrics: remove filer fallback, align with quota enforcement

- Remove fallback to filer Statistics RPC
- Use only master topology for collection info (same as s3.bucket.quota.enforce)
- Updated comments to clarify this runs the same collection logic as quota enforcement
- Simplified code by removing collectBucketSizeFromFilerStats

* use s3a.option.Masters directly instead of querying filer

* address PR comments: fix dashboard overlaps and improve metrics collection

Grafana dashboard fixes:
- Fix overlapping panels 55 and 59 in grafana_seaweedfs.json (moved 59 to y=30)
- Fix grid collision in k8s dashboard (moved panel 72 to y=48)
- Aggregate bucket metrics with max() by (bucket) for multi-instance S3 gateways

Go code improvements:
- Add graceful shutdown support via context cancellation
- Use ticker instead of time.Sleep for better shutdown responsiveness
- Distinguish EOF from actual errors in stream handling

* improve bucket size metrics: multi-master failover and proper error handling

- Initial delay now respects context cancellation using select with time.After
- Use WithOneOfGrpcMasterClients for multi-master failover instead of hardcoding Masters[0]
- Properly propagate stream errors instead of just logging them (EOF vs real errors)

* improve bucket size metrics: distributed lock and volume ID deduplication

- Add distributed lock (LiveLock) so only one S3 instance collects metrics at a time
- Add IsLocked() method to LiveLock for checking lock status
- Fix deduplication: use volume ID tracking instead of dividing by copyCount
  - Previous approach gave wrong results if replicas were missing
  - Now tracks seen volume IDs and counts each volume only once
- Physical size still includes all replicas for accurate disk usage reporting

* rename lock to s3.leader

* simplify: remove StartBucketSizeMetricsCollection wrapper function

* fix data race: use atomic operations for LiveLock.isLocked field

- Change isLocked from bool to int32
- Use atomic.LoadInt32/StoreInt32 for all reads/writes
- Sync shared isLocked field in StartLongLivedLock goroutine

* add nil check for topology info to prevent panic

* fix bucket metrics: use Ticker for consistent intervals, fix pagination logic

- Use time.Ticker instead of time.After for consistent interval execution
- Fix pagination: count all entries (not just directories) for proper termination
- Update lastFileName for all entries to prevent pagination issues

* address PR comments: remove redundant atomic store, propagate context

- Remove redundant atomic.StoreInt32 in StartLongLivedLock (AttemptToLock already sets it)
- Propagate context through metrics collection for proper cancellation on shutdown
  - collectAndUpdateBucketSizeMetrics now accepts ctx
  - collectCollectionInfoFromMaster uses ctx for VolumeList RPC
  - listBucketNames uses ctx for ListEntries RPC

test/s3/iam/test_config.json

@@ -36,7 +36,7 @@
     "sts": {
       "tokenDuration": "15m",
       "issuer": "seaweedfs-sts",
-      "signingKey": "test-sts-signing-key-for-integration-tests"
+      "signingKey": "dGVzdC1zaWduaW5nLWtleS0zMi1jaGFyYWN0ZXJzLWxvbmc="
     },
     "policy": {
       "defaultEffect": "Deny"