IIRC, metallic taste is an off flavor associated with stale (oxidized, old) liquid extract (see BBR Aug 25 / Nov 17 2005 for a list of potential off flavors).
The book
BYO Big Book of Homebrewing (1e, 2015, p 19?) offers a technique for assessing the quality of the extract (liquid or dry) on brew day. I've used this technique once (earlier this year) with a package of LME. I found that the technique works well for assessing the quality of LME before fermenting / packing wort made with stale LME.
"Packaged on" date for the product is a consideration; but be aware that improper storage or shipping may cause liquid extract to go stale quickly (
link to 2016 article).
DME doesn't oxidize as quickly as LME and appears to be more tolerant to hot storage/shipping conditions. I've use 18 month old DME on occasion without issue or concern.