Let the AI gateway read the bodies it is meant to log

Two ceilings had to clear 30 KB prompts, and neither alone was enough:
nginx's client_body_buffer_size, which decides whether the body stays in
memory at all, and ai-proxy's max_request_body_size, which decides whether
the plugin will introspect it. With the first at 8k the body went to a
temp file and kong.request.get_raw_body() returned nil, so log_payloads
and log_statistics were enabled and empty, and no kong_ai_llm_* metric
ever appeared.

256k here against the 8k the domain gateways keep. Affordable because this
gateway is low-traffic; the buffer is per connection carrying a body, so
copying the value to the domain gateways needs that arithmetic redone for
their volume.

Storage cost measured rather than assumed: 18 LLM requests a day at
roughly 60 KB of prompt and completion is 1.1 MiB/day, against 425 MiB/day
of Kong audit records already flowing. My earlier warning that the audit
trail would grow fast was wrong at this volume.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

dev/ai-gateway-kustomization.yaml

@@ -18,6 +18,16 @@ spec:
     substitute:
       NAME: llm-dev
       NAMESPACE: llm
+      # 256k, against the 8k the other domains keep. Prompts here carry the
+      # conversation and the MCP tool definitions and already run to 30 KB;
+      # below this nginx spills the body to a temp file and
+      # kong.request.get_raw_body() returns nil, which is what left ai-proxy's
+      # log_payloads and log_statistics producing nothing at all.
+      #
+      # Affordable precisely because this gateway is low-traffic — the buffer is
+      # per connection carrying a body. Do not copy it to the domain gateways
+      # without doing that arithmetic for their volume.
+      CLIENT_BODY_BUFFER_SIZE: 256k
       HOSTNAME: llm-dev.sttlab.local
     substituteFrom:
       - kind: ConfigMap