out_loki: add ability to add arbitrary http headers and adjust the URI path (#7979)

Following the pattern from prometheus_remote_write, this adds the option to add
arbitrary HTTP headers.

Tested against a Loki endpoint and logging reverse proxy.

Signed-off-by: Alan Richards <alarobric@gmail.com>

Author: alarobric

plugins/out_loki/loki.c

@@ -1515,6 +1515,10 @@ static void cb_loki_flush(struct flb_event_chunk *event_chunk,
     struct flb_connection *u_conn;
     struct flb_http_client *c;
     struct flb_loki_dynamic_tenant_id_entry *dynamic_tenant_id;
+    struct mk_list *head;
+    struct flb_config_map_val *mv;
+    struct flb_slist_entry *key = NULL;
+    struct flb_slist_entry *val = NULL;
 
     dynamic_tenant_id = FLB_TLS_GET(thread_local_tenant_id);
 
@@ -1604,6 +1608,16 @@ static void cb_loki_flush(struct flb_event_chunk *event_chunk,
         flb_http_bearer_auth(c, ctx->bearer_token);
     }
 
+    /* Arbitrary additional headers */
+    flb_config_map_foreach(head, mv, ctx->headers) {
+        key = mk_list_entry_first(mv->val.list, struct flb_slist_entry, _head);
+        val = mk_list_entry_last(mv->val.list, struct flb_slist_entry, _head);
+
+        flb_http_add_header(c,
+                            key->str, flb_sds_len(key->str),
+                            val->str, flb_sds_len(val->str));
+    }
+
     /* Add Content-Type header */
     flb_http_add_header(c,
                         FLB_LOKI_CT, sizeof(FLB_LOKI_CT) - 1,
@@ -1812,6 +1826,12 @@ static struct flb_config_map config_map[] = {
      "Set bearer token auth"
     },
 
+    {
+     FLB_CONFIG_MAP_SLIST_1, "header", NULL,
+     FLB_CONFIG_MAP_MULT, FLB_TRUE, offsetof(struct flb_loki, headers),
+     "Add a HTTP header key/value pair. Multiple headers can be set"
+    },
+
     {
      FLB_CONFIG_MAP_STR, "compress", NULL,
      0, FLB_FALSE, 0,

plugins/out_loki/loki.h

@@ -95,6 +95,9 @@ struct flb_loki {
 
     /* Plugin instance */
     struct flb_output_instance *ins;
+
+    /* Arbitrary HTTP headers */
+    struct mk_list *headers;
 };
 
 #endif