fix: check the meta key budget before scanning every key

The key-count cap exists to catch a whole request body dumped into meta.
That is exactly the shape where the per-key regex runs tens of thousands
of times before the real reason surfaces, so the cheap check goes first.

Also correct two stale docstrings: postgres.py still claimed 22 columns
(it is 24), and _canonical_meta_json promised to raise on non-finite
floats. It is evaluated inside _record's degradation try, so the real
outcome is a warning plus a dropped row -- never an error the caller
sees. What the gate actually buys us is the SQLite side, whose meta is a
TEXT column that would happily store a literal NaN.

src/polygateway/telemetry/postgres.py

@@ -6,7 +6,7 @@
 ① 结构性失败 → warning 一次后永久降级(所有写入短路);
 ② 运行时单条写失败 → 逐条 warning 丢弃,不降级不重试(连接抖动由
 asyncpg 池自恢复;避免浸泡开头一次抖动导致后续全程失遥测)。
-构造不连库(lazy),22 列 schema 与 SQLite 版同名同序。
+构造不连库(lazy),24 列 schema 与 SQLite 版同名同序。
 
 **"结构性"的判据是「确定写不进去」,不是「初始化时出过错」**(issue #9):
 只有建池失败(重试要在业务路径上内联吞掉 connect 超时)与"表确定不存在