fix: http transport ignored when using instrumentation

[lukasmetzner]

Configuration that set a custom HTTP client transport would be ignored. For example in this snippet, the otelhttp.NewTransport would not come into effect.

httpClient := &http.Client{
	Transport: otelhttp.NewTransport(
		http.DefaultTransport,
	),
}

opts := []hcloud.ClientOption{
	hcloud.WithInstrumentation(reg),
	hcloud.WithHTTPClient(httpClient),
}

client := hcloud.NewClient(opts...)

[codecov]

Codecov Report

Attention: Patch coverage is 62.50000% with 3 lines in your changes missing coverage. Please review.

Project coverage is 79.18%. Comparing base (bdb3d26) to head (b1cde02).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
hcloud/internal/instrumentation/metrics.go	50.00%	2 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #642      +/-   ##
==========================================
- Coverage   79.28%   79.18%   -0.10%     
==========================================
  Files          50       50              
  Lines        4407     4411       +4     
==========================================
- Hits         3494     3493       -1     
- Misses        699      703       +4     
- Partials      214      215       +1     

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