proposal: database/sql: add MaxIdleConnections to DBStats

[bonnefoa]

This is a proposal to add MaxIdleConnections to DBStats.

Currently, DBStats exports the following metrics:

type DBStats struct {
    MaxOpenConnections int // Maximum number of open connections to the database.

    // Pool Status
    OpenConnections int // The number of established connections both in use and idle.
    InUse           int // The number of connections currently in use.
    Idle            int // The number of idle connections.

    // Counters
    WaitCount         int64         // The total number of connections waited for.
    WaitDuration      time.Duration // The total time blocked waiting for a new connection.
    MaxIdleClosed     int64         // The total number of connections closed due to SetMaxIdleConns.
    MaxIdleTimeClosed int64         // The total number of connections closed due to SetConnMaxIdleTime.
    MaxLifetimeClosed int64         // The total number of connections closed due to SetConnMaxLifetime.
}

We had issues where our applications were configured with a MaxOpenConnections != MaxIdleConnections, leading to a lot of connection churning to the DB.
MaxOpenConnections is already available in the stats but MaxIdleConnections is missing, making it hard to track affected applications.

[gopherbot]

Change https://go.dev/cl/402334 mentions this issue: database/sql: Add MaxIdleConnections to DBStats

[kardianos]

I'm okay with this proposal.