In a plugin I'm working on, I'm trying to save the current metabolic state...

I thought this would be pretty easy doing something like:

private object OnRunPlayerMetabolism(PlayerMetabolism meta, BaseCombatEntity entity)
{
    BasePlayer basePlayer = entity as BasePlayer;

    // Check if we've already saved the previous state.
    if (onlinePlayers[basePlayer].SavedMetabolism != null) return null;

    onlinePlayers[basePlayer].SavedMetabolism = basePlayer.metabolism.Copy();

    return null;
}​

But I get the following error:

error CS1061: Type `PlayerMetabolism' does not contain a definition for `Copy' and no extension method `Copy' of type `PlayerMetabolism' could be found. Are you missing an assembly reference?

Looking in Rust.Data.dll with JustDecompile, I see:

public PlayerMetabolism Copy()
{
    PlayerMetabolism playerMetabolism = Pool.Get<PlayerMetabolism>();
    this.CopyTo(playerMetabolism);
    return playerMetabolism;
}

public void CopyTo(PlayerMetabolism instance)
{
    instance.health = this.health;
    instance.calories = this.calories;
    instance.hydration = this.hydration;
    instance.heartrate = this.heartrate;
    instance.temperature = this.temperature;
    instance.poison = this.poison;
    instance.radiation_level = this.radiation_level;
    instance.wetness = this.wetness;
    instance.dirtyness = this.dirtyness;
    instance.oxygen = this.oxygen;
    instance.bleeding = this.bleeding;
    instance.radiation_poisoning = this.radiation_poisoning;
    instance.comfort = this.comfort;
    instance.pending_health = this.pending_health;
}​

Is it not possible to make use of these functions from within a plugin?

Maybe in Rust.Data.dll it is in another class?

Nope, it's a part of this class:

public class PlayerMetabolism : IDisposable, Pool.IPooled, IProto​

Check the namespace of this class