How can I grant permission to a user to view Elasticache Metrics in Elasticache console?

If the action in the inline policy was literally: "cloudwatch:" then that won't work. You are missing an action or asterisk (*) after the colon. I was able to recreate this by not having anything after the colon.

Either use: "cloudwatch:GetMetricData" or "cloudwatch:*" to resolve this.

If you did try "cloudwatch:GetMetricData" and it didn't work still ensure that the resource ARN is correct and that you are using an Allow effect in the policy.