feat(client): Add client-side metrics for transfer and RPC operations

[xiaguan]

In this PR, we introduce client side support for both transfer and RPC-related metrics.

We provide interface support: Prometheus-style and human-readable formats. Additionally, we've implemented a background thread for metric printing (disabled by default).

I20250812 17:07:51.023622 117228 client.cpp:1417] Client Metrics Report:
Client Metrics Summary
=== Transfer Metrics Summary ===
Total Read: 5.00 MB
Total Write: 5.00 MB

=== Latency Summary (microseconds) ===
Get: count=103, p95<250μs, max<15000μs
Put: count=4, p95<1500μs, max<15000μs
Batch Get: count=1, p95<125μs, max<15000μs
Batch Put: count=3, p95<5000μs, max<20000μs

=== RPC Metrics Summary ===
GetReplicaList: count=104, p95<150μs, max<200μs
PutStart: count=5, p95<250μs, max<250μs
PutEnd: count=4, p95<200μs, max<1500μs
Remove: count=29, p95<300μs, max<400μs
MountSegment: count=1, p95<125μs, max<7000μs
GetFsdir: count=1, p95<125μs, max<250μs
BatchGetReplicaList: count=1, p95<125μs, max<300μs
BatchPutStart: count=3, p95<250μs, max<500μs
BatchPutEnd: count=3, p95<150μs, max<200μs

[Copilot]

Pull Request Overview

This PR introduces comprehensive client-side metrics for transfer and RPC operations, adding both Prometheus-style and human-readable metric collection interfaces. The changes enable real-time monitoring of client performance with support for configurable background reporting.

Key changes include:

• Implementation of client-side metrics collection for transfer (read/write bytes, latency) and RPC operations
• Addition of background metrics reporting thread with environment variable configuration
• Integration of metrics tracking throughout the client operation flow

Reviewed Changes

Copilot reviewed 9 out of 9 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
client_metric.h	New header defining metric structures for transfer and RPC operations with summary formatting
client.cpp	Integration of metrics collection throughout client operations and background reporting thread
master_client.cpp	RPC call tracking with latency measurement and metric collection
transfer_task.cpp	Transfer operation byte counting and metrics integration
client.h	Client class extension with metrics members and reporting thread management
master_client.h	Header updates for metrics parameter passing
transfer_task.h	Header updates for metrics integration in transfer operations
client_metrics_test.cpp	Comprehensive test suite for all metric functionality
CMakeLists.txt	Build configuration for the new test file

[ykwd]

Overall, this looks good to me. Just two points:

1. Is there a way to skip recording metrics when the metrics feature is disabled? This would allow users to avoid the overhead of metrics if they don’t use this feature.
2. Perhaps we could also add a section in the documentation explaining how to use client metrics.

[xiaguan]

We can now disable metric collection through an environment variable.

Regarding the documentation update, I believe we currently lack an appropriate doc for this. For example, we could include it in a "Mooncake Configuration Guide" or "Mooncake Deployment Guide." I'll update the documentation lately.