refactor(metrics): decouple APMPlus meter uploader

[feng-95]

Summary

• replace the mutable global meter_uploader scan with a thread-safe process registry
• give APMPlus metrics a private MeterProvider, so an existing 0-reader global provider cannot suppress metrics
• deduplicate identical metric destinations while allowing distinct destinations
• route LLM, tool, and skill metrics through one registration path

Dependency and merge order

• fix(tracing): trust preconfigured provider for APMPlus #813 is already merged as ffbf295; its branch remains frozen at 387a856.
• fix(tracing): register exporters added after initialization #810 is now 018cc56dbad7e8fcf192276c7874cd3c252f194d on merged main and must merge before this PR.
• Only commit 1e6c1aa53cc68f5d880564d57e625de5cc8bf645 belongs to the MeterUploader optimization.
• This Draft PR is now mergeable against main.

Verification

Case	Global tracer provider	APMPlus source	Cloud spans	Cloud gen_ai_chat_count
direct env	VeADK	env	4 / 4 unique	1
direct manual	VeADK	manual	4 / 4 unique	1
global env	preconfigured	env	4 / 4 unique	1
global manual	preconfigured	manual	4 / 4 unique	1
global env + manual	preconfigured	env + manual	4 / 4 unique	1
auto-instrument env	auto-instrumented, 0 metric readers	env	5 / 5 unique	1

• relay duplicate counts: 0 for trace spans and metric requests
• targeted tests: 31 passed after the rebase
• Ruff 0.11.12 check and format: passed
• full pytest collection is blocked by optional dependencies missing from the local environment: agentkit.sdk.skills, pypdfium2, and openviking_sdk
• full pre-commit is blocked before hooks run because local Go cannot parse the gitleaks Go 1.23 toolchain directive; Ruff hooks were run directly and passed

Test report

• Feishu: https://bytedance.larkoffice.com/docx/SDbrdibyDohxwixePPWcoVuqnkd
• Local Markdown: /Users/bytedance/go/src/code.byted.org/byteapm/docs-feng/veadk-python/VeADK_APMPlus_MeterUploader_优化测试报告.md