logger/src: Removed Simple Metric
logger/src: Removed Simple Metric

Removed the `SimpleMetric` data type, since it wasn't being used
anywhere in the codebase, either for logging or for testing.

Signed-off-by: Tamio-Vesa Nakajima <b302490a47b9fc3e2bcbd0d5cc21dcbda70b4d31@amazon.com>

firecracker-microvm/firecracker,firecracker-microvm/firecracker,firecracker-microvm/firecracker,firecracker-microvm/firecracker,firecracker-microvm/firecracker