out_datadog: Add support for setting a static hostname

This PR adds support for setting a static hostname in the Datadog output
plugin. This field is analogous to the existing `dd_service` and
`dd_source` configuration options that can be used to set a static
value.

If unset, the default behavior is backwards compatible. This behavior is
to not set an explicity `hostname` field, but if the record has a field
that is detected as the hostname in Datadog (such as `host` or
`syslog.hostname`), it will be picked up.

Closes: #8971

Signed-off-by: Jesse Szwedko <jesse.szwedko@datadoghq.com>

plugins/out_datadog/datadog.c

@@ -247,6 +247,14 @@ static int datadog_format(struct flb_config *config,
                                           ctx->dd_service, flb_sds_len(ctx->dd_service));
         }
 
+        /* dd_hostname */
+        if (ctx->dd_hostname != NULL) {
+            dd_msgpack_pack_key_value_str(&mp_pck,
+                                          FLB_DATADOG_DD_HOSTNAME_KEY,
+                                          sizeof(FLB_DATADOG_DD_HOSTNAME_KEY) -1,
+                                          ctx->dd_hostname, flb_sds_len(ctx->dd_hostname));
+        }
+
         /* Append initial object k/v */
         ind = 0;
         for (i = 0; i < map_size; i++) {
@@ -514,6 +522,14 @@ static struct flb_config_map config_map[] = {
      "The tags you want to assign to your logs in Datadog. If unset, Datadog "
      "will expect the tags in the `ddtags` attribute."
     },
+    {
+     FLB_CONFIG_MAP_STR, "dd_hostname", NULL,
+     0, FLB_TRUE, offsetof(struct flb_out_datadog, dd_hostname),
+     "The host that emitted logs should be associated with. If unset, Datadog "
+     "will expect the host to be set as `host`, `hostname`, or `syslog.hostname` "
+     "attributes. See Datadog Logs preprocessor documentation for up-to-date "
+     "of recognized attributes."
+    },
 
     {
      FLB_CONFIG_MAP_STR, "proxy", NULL,

plugins/out_datadog/datadog.h

@@ -27,6 +27,7 @@
 #define FLB_DATADOG_DEFAULT_PORT      443
 #define FLB_DATADOG_DEFAULT_TIME_KEY  "timestamp"
 #define FLB_DATADOG_DEFAULT_TAG_KEY   "tagkey"
+#define FLB_DATADOG_DD_HOSTNAME_KEY   "hostname"
 #define FLB_DATADOG_DD_SOURCE_KEY     "ddsource"
 #define FLB_DATADOG_DD_SERVICE_KEY    "service"
 #define FLB_DATADOG_DD_TAGS_KEY       "ddtags"
@@ -65,6 +66,7 @@ struct flb_out_datadog {
     int nb_additional_entries;
     flb_sds_t dd_source;
     flb_sds_t dd_service;
+    flb_sds_t dd_hostname;
     flb_sds_t dd_tags;
     flb_sds_t dd_message_key;