Write MongoDB plugin: fix format trunction errors.

Write MongoDB plugin: fix format trunction errors.

When a `size_t` is a 64bit integer, the 16 byte buffer could potentially
overflow. Fix this by using a larger buffer and also collectd's own
`ssnprintf()` function which guarantees to result in a null terminated
string.

```
In function 'wm_create_bson',
    inlined from 'wm_write' at src/write_mongodb.c:229:17:
src/write_mongodb.c:99:32: error: '%zu' directive output may be truncated writing between 1 and 20 bytes into a region of size 16 [-Werror=format-truncation=]
   99 |     snprintf(key, sizeof(key), "%" PRIsz, i);
      |                                ^~~
src/write_mongodb.c: In function 'wm_write':
src/write_mongodb.c:99:33: note: format string is defined here
   99 |     snprintf(key, sizeof(key), "%" PRIsz, i);
```