[out_stackdriver] Support customizing log entry http request field in Stackdriver output plugin

[qingling128]

Bug Report

Describe the bug
Inconsistent Google Cloud Logging LogEntry fields naming convention in the out_stackdriver plugin for http_request.

fluent-bit/plugins/out_stackdriver/stackdriver.h

Line 51 in c785952

#define OPERATION_FIELD_IN_JSON "logging.googleapis.com/operation"

#define OPERATION_FIELD_IN_JSON "logging.googleapis.com/operation"
#define MONITORED_RESOURCE_KEY "logging.googleapis.com/monitored_resource"
#define LOCAL_RESOURCE_ID_KEY "logging.googleapis.com/local_resource_id"
#define DEFAULT_LABELS_KEY "logging.googleapis.com/labels"
#define DEFAULT_SEVERITY_KEY "logging.googleapis.com/severity"
#define DEFAULT_TRACE_KEY "logging.googleapis.com/trace"
#define DEFAULT_LOG_NAME_KEY "logging.googleapis.com/logName"
#define DEFAULT_INSERT_ID_KEY "logging.googleapis.com/insertId"
#define SOURCELOCATION_FIELD_IN_JSON "logging.googleapis.com/sourceLocation"
#define HTTPREQUEST_FIELD_IN_JSON "logging.googleapis.com/http_request"

Expected behavior
Camel cased field names as in https://cloud.google.com/logging/docs/reference/v2/rest/v2/LogEntry

Your Environment

• Version used: Fluent Bit 1.8.8

Additional context
Simply changing logging.googleapis.com/http_request to logging.googleapis.com/httpRequest would likely be considered a breaking change. This either needs to wait for a breaking change release or make Fluent Bit compatible with both values.

[qingling128]

In fact, if we can add something like below (similar to https://github.com/fluent/fluent-bit/blob/master/plugins/out_stackdriver/stackdriver_conf.c#L314-L320)

    tmp = flb_output_get_property("http_request_key", ins);
    if (tmp) {
        ctx->http_request_key = flb_sds_create(tmp);
    }
    else {
        ctx->http_request_key = flb_sds_create(HTTPREQUEST_FIELD_IN_JSON);
    }

and feed that context into the actual place where HTTPREQUEST_FIELD_IN_JSON is being used, it should solve our need.

Braydon from our side is drafting a PR for this.