Report amount of data gathered from environment

Summary:
Completes backport of core PR17270:
https://github.com/bitcoin/bitcoin/pull/17270/commits/d1c02775aa74a0610809ac54bb241ddad61d2d8c

Depends on D6207.

Test Plan:

ninja all check-all

./src/bitcoind

Check in the log that the amount of environment data feeded into RNG is
logged.

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D6209