fix average block time calculation

[ayrat555]

Average block time was recalculated when new blocks were processes. But since block are not added consecutively average block time is getting wronger over time

Changelog

• fix average block time calculation

[coveralls]

Pull Request Test Coverage Report for Build 99f435a8-6a15-40e1-9854-0d5dadcf4fa2

• 5 of 8 (62.5%) changed or added relevant lines in 2 files are covered.
• 14 unchanged lines in 5 files lost coverage.
• Overall coverage decreased (-0.3%) to 83.028%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
apps/explorer/lib/explorer/counters/average_block_time.ex	4	7	57.14%

Files with Coverage Reduction	New Missed Lines	%
apps/indexer/lib/indexer/block/catchup/fetcher.ex	1	65.85%
apps/indexer/lib/indexer/token/fetcher.ex	1	78.57%
apps/indexer/lib/indexer/token_balance/fetcher.ex	2	87.1%
apps/indexer/lib/indexer/block/fetcher.ex	2	91.67%
apps/explorer/lib/explorer/counters/average_block_time.ex	8	47.62%

Totals
Change from base Build b28bfca6-f322-47ca-ba57-bc5965a78876:	-0.3%
Covered Lines:	4447
Relevant Lines:	5356

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💛 - Coveralls