Add metrics to DefaultBlockchain to get TPS and Mgas/s

[ahamlat]

PR description

This PR creates two new prometheus counters gasUsedCounter and numberOfTransactionsCounter.
These new counters will help to create graphs on Transactions per second and Gas used per second.

We can see below the result on Ethereum Mainnet, where the throughput is around 1.26 mgas/s and TPS is slightly less than 14 transactions per second.

Fixed Issue(s)

Thanks for sending a pull request! Have you done the following?

• Checked out our contribution guidelines?
• Considered documentation and added the doc-change-required label to this PR if updates are required.
• Considered the changelog and included an update if required.
• For database changes (e.g. KeyValueSegmentIdentifier) considered compatibility and performed forwards and backwards compatibility tests

Locally, you can run these tests to catch failures early:

• unit tests: ./gradlew build
• acceptance tests: ./gradlew acceptanceTest
• integration tests: ./gradlew integrationTest
• reference tests: ./gradlew ethereum:referenceTests:referenceTests

[macfarla]

prob worth a changelog entry