fix: correct unwrap of http2.Error

[pabigot]

Closes #310

Proposed Changes

Implement Unwrap() correctly.

Checklist

• CHANGELOG.md updated
• Rebased/mergeable
• A test has been added if appropriate
• Tests pass
• Commit messages are in semantic format
• Sign CLA (if not already signed)

[codecov-commenter]

Codecov Report

Merging #311 (10d3bf4) into master (7688764) will decrease coverage by 0.11%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master     #311      +/-   ##
==========================================
- Coverage   90.62%   90.51%   -0.12%     
==========================================
  Files          23       23              
  Lines        2465     2468       +3     
==========================================
  Hits         2234     2234              
- Misses        173      176       +3     
  Partials       58       58              

Impacted Files	Coverage Δ
api/http/error.go	76.19% <0.00%> (-12.70%)	⬇️

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[pabigot]

With this patch this comment can be extended to use:

if errors.Is(err, syscall.ECONNREFUSED) {
    // server unavailable, back off and retry later
}

because it unwraps to this sequence:

2022/02/25 12:40:34.168950 bucket.sensors: [I] write err 1: *http.Error: Post "http://tirzah.pab:8086/api/v2/write?bucket=sensors&org=green-tirzah&precision=us": dial tcp 192.168.65.21:8086: connect: connection refused
2022/02/25 12:40:34.169095 bucket.sensors: [I] write err 2: *url.Error: Post "http://tirzah.pab:8086/api/v2/write?bucket=sensors&org=green-tirzah&precision=us": dial tcp 192.168.65.21:8086: connect: connection refused
2022/02/25 12:40:34.169125 bucket.sensors: [I] write err 3: *net.OpError: dial tcp 192.168.65.21:8086: connect: connection refused
2022/02/25 12:40:34.169147 bucket.sensors: [I] write err 4: *os.SyscallError: connect: connection refused
2022/02/25 12:40:34.169169 bucket.sensors: [I] write err 5: syscall.Errno: connection refused

rather than falling apart at the first unwrap:

2022/02/25 07:05:44.215389 bucket.sensors: [I] write err 1: *http.Error: Post "http://tirzah.pab:8086/api/v2/write?bucket=sensors&org=green-tirzah&precision=us": dial tcp 192.168.65.21:8086: connect: connection refused
2022/02/25 07:05:44.215812 bucket.sensors: [I] write err 2: *errors.errorString: Post "http://tirzah.pab:8086/api/v2/write?bucket=sensors&org=green-tirzah&precision=us": dial tcp 192.168.65.21:8086: connect: connection refused

[vlastahajek]

Thanks for the PR!