middleware.cs

public class HealthCheckMiddleware
{
    private readonly RequestDelegate _next;
    private readonly string _path;

    public HealthCheckMiddleware(RequestDelegate next, string path)
    {
        _next = next;
        _path = path;
    }

    public async Task InvokeAsync(HttpContext context)
    {
        if (context.Request.Path.Value == _path)
        {
            try
            {
                using (var db = new SqlConnection("Database=Oops"))
                {
                    await db.OpenAsync();
                    db.Close();
                }

                context.Response.StatusCode = 200;
                context.Response.ContentLength = 2;
                await context.Response.WriteAsync("UP");
            }
            catch (SqlException)
            {
                context.Response.StatusCode = 400;
                context.Response.ContentLength = 20;
                await context.Response.WriteAsync("SQL Connection Error");
            } 
        }
        else
        {
            await this._next(context);
        }            
    }
}

public static class HealthCheckMiddlewareExtensions
{
    public static IApplicationBuilder UseHealthCheck(this IApplicationBuilder builder, string path)
    {
        return builder.UseMiddleware<HealthCheckMiddleware>(path);
    }
}