Fix json keys for wmbus to be unique

[jedi7]

• Key inst_volume_0 is now unique

Fast fix. Will be nice to find a better solution to provide variable arguments to the append_str.

Previous keys:
"inst_volume_0" : "6.257 m3", "inst_volume_0" : "6.116 m3", "inst_volume_1" : "5.764 m3", "inst_volume_2" : "5.494 m3",
...

New keys:
"inst_volume_0" : "6.257 m3", "inst_volume_m1_0" : "6.116 m3", "inst_volume_m2_1" : "5.764 m3", "inst_volume_m3_2" :
...

[zuckschwerdt]

Thanks, looks great. It might be better to safe-guard
if (key_extra[0] == '\0') and if (pretty_extra[0] == '\0')
as if (!key_extra || !*key_extra) and if (!pretty_extra || !*pretty_extra)
because we do allow at least pretty_key to be NULL safely.
(this !x || !*x is the idiom we typically use for "is empty".)

[jedi7]

updated :) thanks for review