Add a Compute Instance Memory Metric
It would be great if Stackdriver had a compute instance metric which allowed you to track memory usage over time. This would allow users to evaluate whether they are over or under-provisioning memory.
Hi, I spoke with the Stackdriver team, and there are two sets of metrics – one comes from inside the guest, and one comes from outside. We think you may be able to get what you want from this set 1 (the other set is this set 2).
Yes Brian, speaking as a newcomer here, it looks like this is STILL a giant hole in StackDriver, as of Jan 2019. CPU and I/O metrics are of limited value without RAM metrics, most specifically RAM consumed by each process. And not just those super-popular processes like MySQL which have special support in the Agent. It should be a no-brainer for a new user to visually chart 100% of the data provided by a simple console command "ps auxw". Yet it seems StackDriver had made that impossible. RAM consumed by each process is the whole entire reason I tried out StackDriver, and it's not there! The docs imply (vaguely, as you pointed out) that the Agent collects that info, but it doesn't.
A new multi-platform CloudWatch agent is available. You can use a single agent to collect both system metrics and log files from Amazon EC2 instances and on-premises servers.
john smith commented
You can access the Monitoring page for your Computer Classic account from the Services application. On this page, you can select the type of metric you want to view and to specify up to five instances for each metric
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Brian Kessler commented
From the documentation it is not clear what data is returned in  The description says "Agent memory in use" which implies it is the memory used by the agent process and not the memory in use on the instance. Additionally, "The Stackdriver Monitoring agent does not support the following VM instances:
Google App Engine flexible environment instances.
Google Container Engine node instances."
And Google Container Engine node instances are specifically one of our major use cases. There we may be scheduling many different sized containers onto a node and want to know that it is being properly utilized with respect to memory as each container has a requested memory that it requires in order to be scheduled on an instance.