write_open_telemetry plugin: Fix minor memory leak.

write_open_telemetry plugin: Fix minor memory leak.

Error:

```
==171106== 8 bytes in 1 blocks are definitely lost in loss record 58 of 228
==171106==    at 0x48407B4: malloc (vg_replace_malloc.c:381)
==171106==    by 0x492E7F9: strdup (strdup.c:42)
==171106==    by 0x1131F9: cf_util_get_string (configfile.c:1127)
==171106==    by 0x4EA3FD0: ot_config_node (write_open_telemetry.cc:206)
==171106==    by 0x4EA3FD0: ??? (write_open_telemetry.cc:262)
==171106==    by 0x111794: dispatch_block_plugin (configfile.c:464)
==171106==    by 0x1119B9: dispatch_block (configfile.c:508)
==171106==    by 0x11313D: cf_read (configfile.c:1100)
==171106==    by 0x11034E: configure_collectd (collectd.c:356)
==171106==    by 0x1104B6: init_config (collectd.c:406)
==171106==    by 0x121F3F: main (cmd.c:167)
```