Config bools that default to true require more work

[fchikwekwe]

In #954, we discovered that booleans that default to true were being overridden when applying default values. When boolean values are processed by the defaults library, there is not way to distinguish between a false that was intentionally set and a zero boolean value. This results in the default boolean value being used (true in this case) making it not possible for a boolean that defaults to true to be set to false.

We fixed this by setting a pointer to a boolean value so that there are three possible states for a boolean: true, false and nil. If we send a nil boolean to the defaults library it is properly interpreted as unset and can be updated to false.

There is some technical debt leftover from this fix:

• add a getter function for HoneycombLoggerConfig.SamplerEnabled
• add a default-true type in order to avoid hardcoding true values in getter functions for related fields.
• use something (reflect?) to do struct tag lookups to find default values in the getter functions instead of declaring defaults in multiple places

[fchikwekwe]

@robbkidd Already added the GetSamplerEnabled getter function in #969. I suggested in that PR that perhaps getter functions for the other fields on HoneycombLoggerConfig should be added as well.

[fchikwekwe]

So, I did the work of trying to look up struct tags values using reflect in #996, but the consensus is that its wasteful to look up struct tags when we can instead put effort towards creating a defaultTrue type. I will try and work on adding that type.