Add metrics to the roundstate_db

[hbandura]

Add metrics to measure db save time, rlp encoding time, and rlp encoding size, to roundstate_db.

[github-actions]

Coverage from tests in ./e2e_test/... for ./consensus/istanbul/... at commit 8e66bca

coverage: 50.2% of statements across all listed packages

coverage:  60.2% of statements in consensus/istanbul
coverage:  42.7% of statements in consensus/istanbul/announce
coverage:  55.8% of statements in consensus/istanbul/backend
coverage:   0.0% of statements in consensus/istanbul/backend/backendtest
coverage:  24.3% of statements in consensus/istanbul/backend/internal/replica
coverage:  63.9% of statements in consensus/istanbul/core
coverage:  50.0% of statements in consensus/istanbul/db
coverage:   0.0% of statements in consensus/istanbul/proxy
coverage:  64.4% of statements in consensus/istanbul/uptime
coverage:  51.8% of statements in consensus/istanbul/validator
coverage:  79.2% of statements in consensus/istanbul/validator/random

[github-actions]

Test failures:
TestEthersJSCompatibilityDisableAfterGingerbread: e2e_test Checking getExchangeSpenders. spenders = [0x000000000000000000000000000000000000d028] Checking medianRate. numerator = 1000000000000000000000000 denominator = 1000000000000000000000000 Checking gas price minimum. cusdValue = 100000000 Checking getExchangeSpenders. spenders = [0x000000000000000000000000000000000000d028] Checking medianRate. numerator = 1000000000000000000000000 denominator = 1000000000000000000000000 Checking gas price minimum. cusdValue = 100000000 e2e_test.go:742: e2e_test.go:744:
This test report was produced by the test-summary action.  Made with ❤️ in Cambridge.

[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.12% 🎉

Comparison is base (0404e7a) 55.15% compared to head (6a4b726) 55.28%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2040      +/-   ##
==========================================
+ Coverage   55.15%   55.28%   +0.12%     
==========================================
  Files         676      676              
  Lines      114015   114027      +12     
==========================================
+ Hits        62887    63037     +150     
+ Misses      47248    47133     -115     
+ Partials     3880     3857      -23     

Files Changed	Coverage Δ
consensus/istanbul/core/roundstate_db.go	60.26% <100.00%> (+1.61%)	⬆️

... and 28 files with indirect coverage changes

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[palango]

Looks good.

[palango]

@hbandura Should we merge this?

[carterqw2]

Implemented as part of #2184