Metrics improvements

1. memory_*_bytes metrics were not working properly. Somehow the lambda wasn't working correctly, but using partial is. Fine.

2. Instead of calling memory_full_info for every incovation of process_element, just have the gauge callback call it. This avoids the need to keep a local variable, and also reduces the number of times the function is called.

3. Small refactor of how metrics are stored and initialized in EventProcessFunction

Bug: T328925