Godot gives you the option to render strings with DrawString(font, position, text) inside the _Draw method of your Node. But sadly there is no direct way to pass the rotation to this string.

How to rotate your string

Luckily Godot offers a method that transform draw calls and rotate them.

private void _DrawRotatedString(Vector2 position, float rotation, string text)
    {
        // sets this as the new reference for future Draw commands
        DrawSetTransform(position, rotation * (float)(Math.PI/ 180),Vector2.One);
        // Position of string is set to Vector2.Zero because DrawSetTransform already sets position
        DrawString(_dynamicFont, Vector2.Zero, text);
        // resets reference to global origin 0,0 without rotation
        DrawSetTransform(Vector2.Zero, 0,Vector2.One);

    }

Simply call DrawSetTransform(position, rotation, scale) before calling DrawString(). Because I pass in the string rotation as degrees into my _DrawRotatedString function I first have to convert it into radians with: rotation * (float)(Math.PI/ 180). Most Godot functions handling rotation take radians instead of degrees. I also pass in the final position of the string into the DrawSetTransform function because I observed unexpected behaviour when setting the position of DrawSetTransform() to Vector2.zero and the position of DrawString() to the target position. The scale can stay at the default value, which is Vector2.One.

Then I happily draw my string with DrawString(_dynamicFont, Vector2.Zero, text);

Now it is of utmost importance to reset the DrawSetTransform back to DrawSetTransform(Vector2.Zero, 0,Vector2.One); which is the default setting. Otherwise the transform of subsequent draw calls will be shifted and rotated.

For some context here is the entire class including the call of the _DrawRotatedString function and the import of the dynamic font.

using Godot;
using System;
using System.Collections.Generic;
using System.Data;

public class GraphBehaviour : Node2D
{
    DynamicFont _dynamicFont;
    public override void _Ready()
    {
        _dynamicFont = new DynamicFont();
        _dynamicFont.FontData = GD.Load<DynamicFontData>("res://InkbitThree.ttf");
        _dynamicFont.Size = 12;
    }


   public override void _Draw()
   {
        _DrawRotatedString(new Vector2(100,20), 90, "a 90 degree rotated label");
   }

   private void _DrawRotatedString(Vector2 position, float rotation, string text)
   {
        DrawSetTransform(position, rotation * (float)(Math.PI/ 180),Vector2.One);
        DrawString(_dynamicFont, Vector2.Zero, text);
        DrawSetTransform(Vector2.Zero, 0,Vector2.One);
   }

}