A custom health check which checks internal API endpoints. More information: https://blog.vitaliitylyk.com/sitecore-9-3-health-checks/

ApiHealthCheck.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Net.Http;
using System.Threading;
using System.Threading.Tasks;
using Microsoft.Extensions.Diagnostics.HealthChecks;
using Sitecore.Abstractions;

namespace HealthChecks
{
    public class ApiHealthCheck : IHealthCheck
    {
        private static readonly HttpClient httpClient = new HttpClient();
        private static readonly Uri localhostUri = new Uri("http://localhost");
        private readonly BaseSettings settings;
        private static IEnumerable<Uri> uris;

        public ApiHealthCheck(BaseSettings settings)
        {
            this.settings = settings;
        }

        private IEnumerable<Uri> Uris
        {
            get
            {
                if (uris != null)
                {
                    return uris;
                }

                uris = settings
                    .GetPipedListSetting("HealthChecks.Api.Endpoints", Array.Empty<string>())
                    .Select(x => x.Trim())
                    .Where(x => !string.IsNullOrEmpty(x))
                    .Select(x => new Uri(localhostUri, x));

                return uris;
            }
        }

        public async Task<HealthCheckResult> CheckHealthAsync(
            HealthCheckContext context,
            CancellationToken cancellationToken = default)
        {
            foreach (var uri in Uris)
            {
                try
                {
                    var responeMessage = await httpClient.GetAsync(uri, cancellationToken).ConfigureAwait(false);
                    responeMessage.EnsureSuccessStatusCode();
                }
                catch (Exception ex)
                {
                    return HealthCheckResult.Unhealthy($"Endpoint {uri} has returned unsuccessful status code", ex);
                }
            }

            return HealthCheckResult.Healthy();
        }
    }
}