Feature: add prometheus metrics

[0xcb9ff9]

Description

Add metrics

blockchain:

• dogechain_blockchain_gas_avg_price Gas price average (Histogram type)
• dogechain_blockchain_gas_avg_count Gas count average (Histogram type)
• dogechain_blockchain_gas_used Gas used number (Histogram type)
• dogechain_blockchain_block_height block height (Gauge type)
• dogechain_blockchain_block_write_seconds block write to db time (Histogram type)

JSON-RPC:

• dogechain_jsonrpc_requests requests number (Counter type)
• dogechain_jsonrpc_request_errors error request number (Counter type)
• dogechain_jsonrpc_response_seconds request handle time (Histogram type)

Changes include

• Bugfix (non-breaking change that solves an issue)
• Hotfix (change that solves an urgent issue, and requires immediate attention)
• New feature (non-breaking change that adds functionality)
• Breaking change (change that is not backwards-compatible and/or changes current functionality)

Checklist

• I have assigned this PR to myself
• I have added at least 1 reviewer
• I have added the relevant labels
• I have updated the official documentation
• I have added sufficient documentation in code

Testing

• I have tested this code with the official test suite
• I have tested this code manually

[DarianShawn]

A cool feature. 🙌
DAPP developer and node maintainer would found it very useful.

[abrahamcruise321]

ok

[codecov-commenter]

Codecov Report

Merging #205 (3e791eb) into dev (f4bd245) will decrease coverage by 0.21%.
The diff coverage is 22.13%.

@@            Coverage Diff             @@
##              dev     #205      +/-   ##
==========================================
- Coverage   51.31%   51.10%   -0.22%     
==========================================
  Files         115      117       +2     
  Lines       17345    17475     +130     
==========================================
+ Hits         8901     8931      +30     
- Misses       7685     7784      +99     
- Partials      759      760       +1     

Impacted Files	Coverage Δ
jsonrpc/jsonrpc.go	18.87% <7.14%> (-0.80%)	⬇️
jsonrpc/metrics.go	17.77% <17.77%> (ø)
blockchain/metrics.go	23.52% <23.52%> (ø)
blockchain/blockchain.go	49.70% <31.57%> (-0.42%)	⬇️
blockchain/testing.go	60.28% <100.00%> (+0.38%)	⬆️
protocol/sync_peer.go	70.00% <0.00%> (+1.00%)	⬆️

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[DarianShawn]

Great job.
Thank you for your important contribution to this. 🙏