[Store] Structure ClientMetricConfig environment settings

[bitborne]

Description

This PR continues the staged convergence of mooncake-store environment configuration for client metrics.

• Group MC_STORE_CLIENT_METRIC, MC_STORE_CLIENT_METRIC_INTERVAL, and MC_STORE_CLIENT_METRIC_BANDWIDTH definitions under ClientMetricEnvironmentVariables.
• Add ClientMetricConfig::FromEnvironment() to own environment reads and parsing before ClientMetric construction.
• Preserve the existing defaults, accepted values, invalid and empty value fallbacks, diagnostics, and the disabled-metrics short-circuit.
• Add focused tests for default, valid, invalid, empty, negative, overflow, zero, logging, and environment restoration behavior.

PR #1538 also touches client metric environment handling as part of a broad cross-component migration. This PR is intentionally limited to the current grouped environment-definition and configuration-object pattern, without changing existing client metric behavior.

Module

• Transfer Engine (mooncake-transfer-engine)
• Mooncake Store (mooncake-store)
• Reshard (mooncake-reshard)
• Mooncake EP (mooncake-ep)
• Mooncake PG (mooncake-pg)
• Integration (mooncake-integration)
• P2P Store (mooncake-p2p-store)
• Python Wheel (mooncake-wheel)
• Common (mooncake-common)
• Mooncake RL (mooncake-rl)
• CI/CD
• Docs
• Other

Type of Change

• Bug fix
• New feature
• Refactor
• Breaking change
• Documentation update
• Performance improvement
• Other

How Has This Been Tested?

Test commands:

cmake --build build --target client_metric_config_test client_metrics_test -j32
ctest --test-dir build -R '^(client_metric_config_test|client_metrics_test)$' --output-on-failure
pre-commit run --files mooncake-common/include/environment_variables.h mooncake-store/include/client_metric.h mooncake-store/src/CMakeLists.txt mooncake-store/src/client_metric.cpp mooncake-store/src/config/client_metric_config.cpp mooncake-store/tests/CMakeLists.txt mooncake-store/tests/client_metric_config_test.cpp mooncake-store/tests/client_metrics_test.cpp

Test results:

• Unit tests pass
• Integration tests pass (if applicable)
• Manual testing done (describe below)

Checklist

• I have performed a self-review of my own code
• I have formatted my code using ./scripts/code_format.sh
• I have run pre-commit on the files changed in this PR and all hooks pass
• I have updated the documentation (if applicable)
• I have added tests to prove my changes are effective
• For changes >500 LOC: I have filed an RFC issue

AI Assistance Disclosure

• No AI tools were used
• AI tools were used (specify below)

Codex was used to inspect the existing configuration patterns, implement the focused refactor and tests, and run review and verification checks. I reviewed the final diff and can explain the changes end to end.