[collectd 6] Missing "StoreRates" support for "write_http" plugin (regression from v5)

[eero-t]

• Version of collectd: v6-rc

Expected behavior

write_http plugin is documented to support StoreRates option, and does try to use it:

• https://github.com/collectd/collectd/blob/collectd-6.0/src/collectd.conf.pod#L11375
• https://github.com/collectd/collectd/blob/collectd-6.0/src/write_http.c#L438

Actual behavior

Compiler warns that store_rates args is not used in format_json_metric_family(): https://github.com/collectd/collectd/blob/collectd-6.0/src/utils/format_json/format_json.c#L219

Notes

• This regression from v5 functionality was introduced in eeb6ffa
• write_http plugin is only user of that functionality, write_log sets it false:

$ git grep "format_json_metric_family *(" src/*.c
src/write_http.c:      format_json_metric_family(&cb->send_buffer, fam, cb->store_rates);
src/write_log.c:  int status = format_json_metric_family(&buf, fam, /* store rates = */ false);