MINOR: trace: add a per-source helper to pre-fill the context

Now sources which want to do it can provide a helper that can pre-fill
some fields in the context based on their knowledge (e.g. mux streams).

include/haproxy/trace-t.h

@@ -166,6 +166,8 @@ struct trace_source {
 	                   const struct trace_source *src,
 	                   const struct ist where, const struct ist func,
 	                   const void *a1, const void *a2, const void *a3, const void *a4);
+	void (*fill_ctx)(struct trace_ctx *ctx, const struct trace_source *src,
+	                 const void *a1, const void *a2, const void *a3, const void *a4);
 	uint32_t arg_def;        // argument definitions (sum of TRC_ARG{1..4}_*)
 	const struct name_desc *lockon_args; // must be 4 entries if not NULL
 	const struct name_desc *decoding;    // null-terminated if not NULL

src/trace.c

@@ -123,6 +123,9 @@ int __trace_enabled(enum trace_level level, uint64_t mask, struct trace_source *
 	if (src->arg_def & TRC_ARGS_APPCTX)
 		ctx.appctx = trace_pick_arg(src->arg_def & TRC_ARGS_APPCTX, a1, a2, a3, a4);
 
+	if (src->fill_ctx)
+		src->fill_ctx(&ctx, src, a1, a2, a3, a4);
+
 #ifdef USE_QUIC
 	if (ctx.qc && !ctx.conn)
 		ctx.conn = ctx.qc->conn;