feat: record sampling parameters in telemetry (port 20 to 21 fields)

Each of the three emitter entry points has a pinned meaning: only the
attempt path has an effective source, so only it merges extra_body.

src/polygateway/telemetry/postgres.py

@@ -41,7 +41,8 @@ CREATE TABLE IF NOT EXISTS llm_calls (
     cost               DOUBLE PRECISION,
     created_at         TIMESTAMPTZ NOT NULL DEFAULT now(),
     cached_prompt_tokens INTEGER,
-    model_reported       TEXT
+    model_reported       TEXT,
+    sampling             TEXT
 );
 """
 
@@ -49,6 +50,7 @@ CREATE TABLE IF NOT EXISTS llm_calls (
 _BACKFILL = (
     ("cached_prompt_tokens", "ALTER TABLE llm_calls ADD COLUMN cached_prompt_tokens INTEGER"),
     ("model_reported", "ALTER TABLE llm_calls ADD COLUMN model_reported TEXT"),
+    ("sampling", "ALTER TABLE llm_calls ADD COLUMN sampling TEXT"),
 )
 
 # 探测现有列;尊重 search_path(to_regclass 按当前 search_path 解析)
@@ -78,6 +80,7 @@ _COLUMNS = (
     "cost",
     "cached_prompt_tokens",
     "model_reported",
+    "sampling",
 )
 
 _INSERT = (