Most kinds of glue adhere readily to metal surfaces (and other hard surfaces) and are not usually easy to get off the surfaces using soap and water alone. However, no matter if the glue is sticker adhesive, craft glue, or even a stronger glue (such as tile adhesive or wood glue), you'll be able to remove glue from metal with easily with the right product. In addition to making the job less labour intensive, these products will work quickly.
Open a window for ventilation and put on protective gloves; household rubber gloves should be sufficient. Apply adhesive remover to the glue. Some products spray on, others are sponged on or dabbed on with an applicator or cotton swab.
Wait for the remover to work. This may take from five to fifteen minutes. How much time it takes too soften the glue depends on the type of glue, the thickness, and , possibly, the product you use.
Scrape off the glue with a plastic scraper. Pick up the soft clump of glue with a paper towel for disposal.
Pour mild soap and warm water on a sponge. Wash off the metal surface. This should remove any remaining adhesive along with the adhesive remover. If any glue remains, reapply adhesive remover.
Rinse the sponge and wipe down the metal to rinse off soap residue. Use a cloth to remove all remaining moisture on the metal; if you don't, you're inviting rust.
As an alternative to adhesive removers, you can try WD-40 on the glue. It will sometimes loosen it enough to scrape off. Any piece of hard plastic will work as a plastic scraper---it may not be necessary to go out and purchase one.
Some solvents will remove paint. Use these with caution on painted metal. It is prudent to test an area first to see how it reacts to the remover. Always work in a well-ventilated room when working with adhesive removers. If you get any on your skin and clothes, wash immediately and/or follow the safety directions on the bottle. In addition, some solvent adhesive removers are flammable---do not use these around open flames.