update metal ip resources to retrieve metro from facility if metro is null

[ocobles]

• equinix_metal_reserved_ip_block:
  Public IPv4 blocks auto-assigned (management subnets) to a device cannot be retrieved. This PR updates failing test and improve documentation.
• equinix_metal_reserved_ip_block, equinix_metal_ip_block_ranges:
  metro is nil for some IP blocks. For those cases metro will be retrieved from the facilitiy
• update docs

Fix #243

[codecov-commenter]

Codecov Report

Base: 60.23% // Head: 60.04% // Decreases project coverage by -0.19% ⚠️

Coverage data is based on head (8d4fe0b) compared to base (3850780).
Patch coverage: 76.00% of modified lines in pull request are covered.

❗ Current head 8d4fe0b differs from pull request most recent head ae47310. Consider uploading reports for the commit ae47310 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #265      +/-   ##
==========================================
- Coverage   60.23%   60.04%   -0.20%     
==========================================
  Files          74       74              
  Lines       12745    12758      +13     
==========================================
- Hits         7677     7660      -17     
- Misses       4769     4794      +25     
- Partials      299      304       +5     

Impacted Files	Coverage Δ
equinix/data_source_metal_ip_block_ranges.go	80.48% <63.63%> (-2.13%)	⬇️
equinix/data_source_metal_precreated_ip_block.go	79.16% <77.77%> (-2.35%)	⬇️
equinix/data_source_metal_reserved_ip_block.go	85.43% <100.00%> (+0.19%)	⬆️
equinix/resource_metal_reserved_ip_block.go	83.25% <100.00%> (-0.67%)	⬇️
equinix/data_source_metal_virtual_circuit.go	96.36% <0.00%> (-3.64%)	⬇️
equinix/resource_metal_device.go	68.82% <0.00%> (-2.55%)	⬇️
equinix/resource_metal_spot_market_request.go	56.53% <0.00%> (-1.16%)	⬇️
equinix/helpers_device.go	67.89% <0.00%> (-0.53%)	⬇️
... and 1 more

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