Add MsgJsonFormat custom event formatter that outputs JSON with _msg as
the first field, required by VictoriaLogs for full-text search. HTTP
middleware stores interpolated "METHOD /path" in thread-local buffer
for the formatter to read on span-close events.
- Delete admin metrics dashboard page (admin/metrics/page.tsx)
- Delete LineChart component (used only by metrics)
- Remove "指标监控" nav link from Sidebar
- Remove getMetrics/exportMetricsCsv from admin-rpc.ts client
- Remove /api/admin/metrics and /api/admin/metrics/export HTTP routes
from adminrpc (was also leaking metrics via HTTP)
- Remove metrics RPC methods (get_metrics, export_metrics_csv) from
adminrpc gRPC server and their helper functions
- Remove spawn_redis_metrics_flusher from app/main.rs
- Remove redis_metrics module from observability crate
- Remove redis/deadpool-redis deps from observability Cargo.toml