Export volume name to tikv metrics

[polyrabbit]

A TiKV cluster can serve metadata for multiple JuiceFS volumes. This PR exposes each JuiceFS volume name as a tenant identity in TiKV request-source metrics, which allows operators to monitor:

• Request rate per volume with tikv_grpc_request_source_counter_vec
• Average TiKV request latency per volume with tikv_grpc_request_source_duration_vec/tikv_grpc_request_source_counter_vec

When a TiKV cluster is under pressure, this makes it easier to quickly identify which volumes contribute most of the request traffic.

Following dashboard screenshot shows real per-volume source labels after this change:

[codecov]

Codecov Report

❌ Patch coverage is 57.14286% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 56.99%. Comparing base (75730ca) to head (83c096b).
⚠️ Report is 14 commits behind head on main.

Files with missing lines	Patch %	Lines
pkg/meta/tkv_tikv.go	57.14%	3 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #7480       +/-   ##
===========================================
+ Coverage   21.95%   56.99%   +35.04%     
===========================================
  Files          31      180      +149     
  Lines       24747    59681    +34934     
===========================================
+ Hits         5433    34018    +28585     
- Misses      18719    22052     +3333     
- Partials      595     3611     +3016     

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