Add "const" modifier to "char *" parameters that don't modify it

[tarruda]

Functions that create/modify metrics don't modify their char * parameters, so it would be good to add const modifier to let the compiler know callers don't expect it to be modified.

[edsiper]

used to be a practice, but at the moment that might trigger lot of warnings in Fluent Bit caller

[fredrik-jansson-se]

Integrating with cmetrics from rust (bindgen), and it would had been great with const modifers. As of now I have to jump through hoops to pass c-strings to functions that could take const char*.