GODRIVER-2689 (POC) Simplify the "readpref" API

[prestonvasquez]

GODRIVER-2689

Summary

Background & Motivation

[mongodb-drivers-pr-bot]

API Change Report

./v2/event

incompatible changes

##ServerDescription.Tags: changed from ./v2/tag.Set to ./v2/mongo/readpref.TagSet

./v2/mongo/readpref

incompatible changes

(*ReadPref).MaxStaleness: changed from func() (time.Duration, bool) to func() *time.Duration
(*ReadPref).Mode: removed
##(*ReadPref).TagSets: changed from func() []./v2/tag.Set to func() []TagSet
ErrInvalidTagSet: removed
Mode.IsValid: removed
Mode.String: removed
Mode: changed from uint8 to string
Nearest: changed from func(...Option) *ReadPref to func(...*Builder) *ReadPref
NearestMode: changed from Mode to untyped string
New: changed from func(Mode, ...Option) (*ReadPref, error) to func(Mode, ...*Builder) (*ReadPref, error)
Option: removed
PrimaryMode: changed from Mode to untyped string
PrimaryPreferred: changed from func(...Option) *ReadPref to func(...*Builder) *ReadPref
PrimaryPreferredMode: changed from Mode to untyped string
Secondary: changed from func(...Option) *ReadPref to func(...*Builder) *ReadPref
SecondaryMode: changed from Mode to untyped string
SecondaryPreferred: changed from func(...Option) *ReadPref to func(...*Builder) *ReadPref
SecondaryPreferredMode: changed from Mode to untyped string
WithHedgeEnabled: removed
WithMaxStaleness: removed
WithTagSets: removed
WithTags: removed

compatible changes

Builder: added
NewTagSet: added
NewTagSetFromMap: added
NewTagSetsFromMaps: added
Options: added
ReadPref.Mode: added
Tag: added
TagSet: added

./v2/tag

incompatible changes

package removed

./v2/x/mongo/driver/description

incompatible changes

##SelectedServer.Tags: changed from ./v2/tag.Set to ./v2/mongo/readpref.TagSet
##Server.Tags: changed from ./v2/tag.Set to ./v2/mongo/readpref.TagSet

[prestonvasquez]

The current pattern prevents users from creating invalid read preferences, for example:

ms := 1 * time.Second
rp := readpref.ReadPref{
    Mode: readpref.PrimaryMode, 
    MaxStaleness: &ms, 
}

Concerning server selection:

If the mode field is “primary”, the tags, maxStalenessSeconds, and hedge fields MUST be absent.

Concerning max staleness:

A connection string combining a positive maxStalenessSeconds with read preference mode “primary” MUST be considered invalid; this includes connection strings with no explicit read preference mode.

The only driver I've investigated that doesn't follow the pattern of validating in the read preference constructor is C (compared to Node, Python, PHP). C uses a _mongoc_read_prefs_validate function to validate this behavior.