out_es: Allow Logstash_Prefix_Key with prefix

The option Logstash_Prefix_Key should be a record accessor.

There is a convenience feature that automatically prefixes a $ when the value is just a static string (since that is expected to be set as Logstash_Prefix instead).

However, record accessor can contain the $ not just at the beginning. One example would be
  containers-$kubernetes['namespace_name']

This patch thus makes the autodetection look for a $ in the whole value, not just the first character.

Signed-off-by: Jan-Philipp Litza <[email protected]>

plugins/out_es/es_conf.c

@@ -326,7 +326,7 @@ struct flb_elasticsearch *flb_es_conf_create(struct flb_output_instance *ins,
     }
 
     if (ctx->logstash_prefix_key) {
-        if (ctx->logstash_prefix_key[0] != '$') {
+        if (strchr(ctx->logstash_prefix_key, '$') == NULL) {
             len = flb_sds_len(ctx->logstash_prefix_key);
             buf = flb_malloc(len + 2);
             if (!buf) {