Provide a way to convert ErrorCode enum name to a string

[james-johnston-thumbtack]

Description

The kafka.ErrorCode type only provides a String function that converts it to a human-readable string description of the error. But I would like to get the actual error code constant name. The motivation is to use it as a low-cardinality and concise string for monitoring purposes (storing as InfluxDB tag) and thus be able to view/aggregate by various error codes in Grafana. (While the current implementation of ErrorCode.String may also have low cardinality, it is longer than I want for my purposes.) For example:

var code kafka.ErrorCode = kafka.ErrInvalidRequest
fmt.Println(code.Name())

would print either ErrInvalidRequest or INVALID_REQUEST (derived from RD_KAFKA_RESP_ERR_INVALID_REQUEST). I don't really have a preference on which one.

This could be accomplished by calling either rd_kafka_err2name to get the INVALID_REQUEST string. Or alternatively, code generate a Name function that contains a giant "switch" statement that returns the appropriate string (e.g. similar to the hand-written func (p Pill) String() string example at https://blog.golang.org/generate )

[edenhill]

code.Name() -> err2name() seems to be the easiest alternative.

Suggest using the same names as in generated_errors.go, see the generator for how they are constructed.