ManagementPortMiddleware should support CF_INSTANCE_PORTS

[TimHess]

Is your feature request related to a problem? Please describe.

When using cf register-metrics-endpoint APP-NAME PATH --internal-port PORT, HTTP requests aren't actually sent to the port specified, but rather to a proxy port that can be found by parsing the environment variable CF_INSTANCE_PORTS.

Example value of CF_INSTANCE_PORTS from a sample app that is configured for port 8091:

[{\"internal\":8080,\"external_tls_proxy\":61004,\"internal_tls_proxy\":61001},{\"internal\":8080,\"internal_tls_proxy\":61443},{\"internal\":8091,\"external_tls_proxy\":61005,\"internal_tls_proxy\":61002},{\"internal\":2222,\"external_tls_proxy\":61058,\"internal_tls_proxy\":61003}]

Describe the solution you'd like

Steeltoe should evaluate CF_INSTANCE_PORTS and consider one or both of the proxy ports to effectively count as requests to the management port.

Prototype

private static bool IsRequestAllowed(HttpRequest request, ManagementOptions managementOptions, ILogger logger)
{
    if (int.TryParse(managementOptions.Port, CultureInfo.InvariantCulture, out int managementPort) && managementPort > 0)
    {
        bool isManagementPath = request.Path.StartsWithSegments(managementOptions.Path);
        bool isManagementScheme = managementOptions.SslEnabled ? request.Scheme == Uri.UriSchemeHttps : request.Scheme == Uri.UriSchemeHttp;
        bool isManagementPort = request.Host.Port == managementPort;

        string? instancePorts = Environment.GetEnvironmentVariable("CF_INSTANCE_PORTS");

        if (isManagementPort || string.IsNullOrEmpty(instancePorts))
        {
            return isManagementPath ? isManagementScheme && isManagementPort : !isManagementScheme || !isManagementPort;
        }

        var portMappings = JsonSerializer.Deserialize<List<PortMapping>>(instancePorts);

        if (portMappings != null)
        {
            isManagementPort = portMappings.Exists(mapping => mapping.Internal == managementPort && (request.Host.Port == mapping.ExternalTlsProxy || request.Host.Port == mapping.InternalTlsProxy));
        }

        return isManagementPath ? isManagementScheme && isManagementPort : !isManagementScheme || !isManagementPort;
    }

    return true;
}

private sealed class PortMapping
{
    [JsonPropertyName("internal")]
    public int Internal { get; set; }
    [JsonPropertyName("external_tls_proxy")]
    public int? ExternalTlsProxy { get; set; } // Nullable, since not all objects have this property
    [JsonPropertyName("internal_tls_proxy")]
    public int? InternalTlsProxy { get; set; } // Nullable, since not all objects have this property
}