Use MemAvailable rather than MemFree for metrics #2954

Job	Run time
photonclient-check-lint	35s
photonclient-build	1m 1s
photon-build-all	5m 26s
Build Examples	4m 30s
Photonlib - Build Docker - Athena	9m 8s
Photonlib - Build Host - Win64	8m 46s
wpiformat	23s
Photonlib - Build Docker - Raspbian	12m 43s
photonserver-build-offline-docs	1m 30s
Photonlib - Build Host - Linux	4m 57s
Photonlib - Build Docker - Aarch64	10m 11s
photonserver-check-lint	41s
Build fat JAR - Win64	3m 19s
Build fat JAR - macOS	4m 36s
Build fat JAR - Linux	2m 4s
Build fat JAR - macOSArm	3m 11s
Build fat JAR - LinuxArm32	2m 6s
Build fat JAR - LinuxArm64	2m 59s
Build image - ${{ matrix.image_url }}	0s
photon-release	0s
	1h 18m 6s

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