[Observability] add more blockchain metrics

[longbowlu]

including but not limited to the following:

• num of distinct addresses that have sent txes
• num of live objects
• num of distinct object ID's (i.e., # live objects + # deleted objects)
• num of Move package objects
• total size of object DB
• total size of locks DB
• total size of tx log
• average tx size
• average object size
• max tx size
• max object size

[longbowlu]

I don't like my current draft solution. A more robust approach is to store these metrics on DB (e.g. IndexStore or it can use a better name), and we have a period tokio task to report the metrics.
@oxade is offering to help me facilitate some variable-ish value storage in rocksdb, with atomicity & read consistency. I will resume the work once that feature is ready

[longbowlu]

had an off-line discussion with @oxade and @velvia. The conclusion is @velvia will take this task on