MINOR: http-htx: Add a function to retrieve the headers size of an HTX message

http_get_hdrs_size() function may now be used to get the bytes held by headers
in an HTX message. It only works if the headers were not already
forwarded. Metadata are not counted here.
diff --git a/src/http_htx.c b/src/http_htx.c
index 587b9a5..f2f0a0c 100644
--- a/src/http_htx.c
+++ b/src/http_htx.c
@@ -72,6 +72,24 @@
 	return htx_get_blk_ptr(htx, blk);
 }
 
+/* Returns the headers size in the HTX message */
+size_t http_get_hdrs_size(struct htx *htx)
+{
+	struct htx_blk *blk;
+	size_t sz = 0;
+
+	blk = htx_get_first_blk(htx);
+	if (!blk || htx_get_blk_type(blk) > HTX_BLK_EOH)
+		return sz;
+
+	for (; blk; blk = htx_get_next_blk(htx, blk)) {
+		sz += htx_get_blksz(blk);
+		if (htx_get_blk_type(blk) == HTX_BLK_EOH)
+			break;
+	}
+	return sz;
+}
+
 /* Finds the first or next occurrence of header <name> in the HTX message <htx>
  * using the context <ctx>. This structure holds everything necessary to use the
  * header and find next occurrence. If its <blk> member is NULL, the header is