I have installed the Xcode Command Line Tools with xcode-select --install and they have been working fine for me. I am now trying to use Metal, and get this error:
xcrun: error: unable to find utility "metal", not a developer tool or in PATH
The macOS SDK appears to be present at /Library/Developer/CommandLineTools/SDKs/MacOSX10.13.sdk but I can't find a metal utility either there on anyway else in the tools' directory.
An issue was filed with gfx-rs and put down to environment misconfiguration. It doesn't really look like this was the case, since the resolution was to point away from the Command Line Tools at the full application. An more knowledgeable issue was filed with Unreal Engine which implies that this is not possible with a CLI-only install.
Is it possible to use Metal without a full Xcode install?
I don't think so. The Metal command-line tools are located at
$XCODE_PATH/Contents/Developer/Platforms/$PLATFORM_NAME.platform/usr/bin
but are not in the standalone Command Line Tools download, probably because they're regarded as SDK content.
You could file an enhancement request Radar asking for the Metal tools to be distributed in this way; it's worth a shot.