Add Dns Resolve Host Count HealthCheck #684

[CarlosLanderas]

Added new DNS healthchecks in Network package to allow configuring a target host and the minimum and maximum IP addresses that should be resolved for that host name.

services
 .AddHealthChecks()
 .AddDnsResolveHostCountHealthCheck(setup =>
  {
     setup.AddHost("yourdomain.com", minHosts: 1, maxHosts: 3);
     setup.AddHost("otherdomain.com", minHosts: 1, maxHosts: 1);
  });

What this PR does / why we need it:

Which issue(s) this PR fixes: #684

Please reference the issue this PR will close: #[issue number]

Special notes for your reviewer:

Does this PR introduce a user-facing change?:

Please make sure you've completed the relevant tasks for this PR, out of the following list:

• Code compiles correctly
• Created/updated tests
• Unit tests passing
• End-to-end tests passing
• Extended the documentation
• Provided sample for the feature

[bakgerman]

yes that looks great!

[bakgerman]

So, if we don't care about max, only min, we could put MAX_INT as the value...or we could make max_hosts a nullable int, etc...

[CarlosLanderas]

@bakgerman changed.

Signature:

  public DnsResolveCountOptions AddHost(string hostName, int minHosts, int? maxHosts)
   {
       if (!HostRegistrations.TryGetValue(hostName, out var _))
       {
         HostRegistrations.Add(hostName, (minHosts, maxHosts));
       }

       return this;
    }

Check:

 if (totalAddresses < minHosts || (maxHosts.HasValue && totalAddresses > maxHosts.Value))
 {
   failedResolutions.Add((entry.Key, totalAddresses));
 }

[sungam3r]

Suggested change

	if (totalAddresses < minHosts || (maxHosts.HasValue && totalAddresses > maxHosts.Value))
	if (totalAddresses < minHosts || totalAddresses > maxHosts)

[CarlosLanderas]

mmm. maxHosts.Value will crash if it has no value?

[sungam3r]

Sorry, my typo, fixed.