exporter: parallelize metrics generation and refresh in background

[qingqiuhe]

Summary

This PR brings over the two main optimizations that exist in our fork but are still missing from GitHub main:

1. Parallelize exporter metric collection loops with bounded concurrency.
2. Replace on-demand /metrics generation with background periodic refresh.

What changed

• parallelized device-level and container-level metrics collection in MetricsGenerator with errgroup.SetLimit(...)
• added exporter_concurrency_limit config with default 16
• moved metrics generation off the /metrics request path and into a background ticker
• added metrics_generate_interval config with default 15s
• exposed both settings in server/config/config.yaml and the Helm chart values/configmap
• promoted golang.org/x/sync to a direct dependency in server/go.mod

Why

On the current GitHub main branch:

• exporter collection is still largely serial inside the device/container loops
• /metrics still triggers synchronous GenerateMetrics(...) on every scrape
• the new concurrency and refresh interval settings are not wired through config/Helm

This means scrape latency can grow with cluster size, and Prometheus requests stay coupled to live collection latency.

Validation

• cd server && go test ./internal/exporter ./internal/server ./cmd/server

Note: cd server && go test ./... still fails on an existing unrelated issue in internal/provider/util/util.go:332 (fmt.Sprintf argument mismatch), which is outside the scope of this PR.

[hami-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: qingqiuhe

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[hami-robot]

Welcome @qingqiuhe! It looks like this is your first PR to Project-HAMi/HAMi-WebUI 🎉