feat: add router_rebuild_min_interval to reduce CPU spikes from frequ…

[membphis]

Description

When routes change frequently (e.g., 20-50 changes/sec with 5000+ total routes), the data plane rebuilds the radixtree router on every single change, causing significant CPU spikes (up to 80%) and temporary TPS drops.

This PR adds a configurable router_rebuild_min_interval option that sets a minimum time interval (in seconds) between HTTP router rebuilds. When the interval has not elapsed since the last rebuild, subsequent route changes are acknowledged but the router rebuild is deferred, dramatically reducing CPU usage during bulk route updates.

Configuration

apisix:
  router:
    http: radixtree_host_uri
    ssl: radixtree_sni
    router_rebuild_min_interval: 0  # default: 0 (rebuild immediately, current behavior)

Expected Impact

Scenario (30s duration)	min_interval: 0	min_interval: 1	min_interval: 5
20 changes/sec	600 rebuilds	~31 rebuilds	~7 rebuilds
50 changes/sec	1500 rebuilds	~31 rebuilds	~7 rebuilds
1 change/3sec	10 rebuilds	10 rebuilds	10 rebuilds

Changes

• conf/config.yaml.example: Added router_rebuild_min_interval config option
• apisix/cli/config.lua: Default value 0
• apisix/router.lua: Reads config and passes to HTTP router modules (+3 lines)
• apisix/http/router/radixtree_uri.lua: Min interval check before rebuild
• apisix/http/router/radixtree_host_uri.lua: Min interval check before rebuild
• apisix/http/router/radixtree_uri_with_parameter.lua: Min interval check before rebuild
• t/router/router-rebuild-min-interval.t: Test cases covering default behavior, skip behavior, and deferred rebuild

Backward Compatibility

Default value is 0, preserving the existing behavior of rebuilding immediately on every route change. No existing configurations are affected.

[moonming]

Architectural Note (self-review):

This PR (#13073) and #12826 (incremental router compilation by @apaloleg) address the same problem — CPU spikes from frequent router rebuilds — with different approaches:

• This PR: router_rebuild_min_interval — throttle/debounce rebuilds (quick win, low risk)
• feat: incremental compilation routers #12826: Incremental compilation — only rebuild affected parts of the radix tree (fundamental fix, higher complexity)

Proposed path forward:

1. Merge this throttling approach first as an immediate relief (low risk, easy to understand)
2. Continue developing incremental compilation as a longer-term optimization
3. The two approaches are complementary — throttling reduces frequency, incremental reduces cost per rebuild

Will coordinate with @apaloleg on #12826 to align timelines.

[github-actions]

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[github-actions]

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