allow fixed heap resource monitor's max_heap_size_bytes to be configurable via runtime

[ramaraochavali]

Currently fixed heap resource monitor's max_heap_size_bytes is in bootstrap and can only take static value. For deployments like Istio, where we set limits via annotations, it would be good if we can allow this value from runtime variable also so that we can set appropriate run time configuration (the same value as we set in the annotation)

[phlax]

cc @eziskind @htuch @nezdolik

[github-actions]

This issue has been automatically marked as stale because it has not had activity in the last 30 days. It will be closed in the next 7 days unless it is tagged "help wanted" or "no stalebot" or other activity occurs. Thank you for your contributions.

[ramaraochavali]

not stale @phlax Can you please mark it help wanted?