Add Memory Available metric to memory monitoring

[crohr]

Summary

• Adds mem_available_percent metric alongside the existing mem_used_percent when memory monitoring is enabled
• This provides a more accurate view of available memory that accounts for shared memory like tmpfs, which was skewing data when only using "Memory Used"

Changes

• Added new available_percent measurement to the memory metric type in GetMeasurements()
• The CloudWatch agent will now collect both mem_used_percent and mem_available_percent
• Both metrics will be displayed in the metrics summary with ASCII charts

Test plan

• Deploy to a test runner with metrics: memory enabled
• Verify both "Memory Used" and "Memory Available" charts appear in the post-execution metrics summary
• Confirm the Memory Available values account for shared memory (tmpfs)

Closes #22