feat: Trace log heartbeat (#127)

Author: ChrisDoernen

src/EventSourcingDb/Client.cs

@@ -350,6 +350,7 @@ public async IAsyncEnumerable<Event> ObserveEventsAsync(
                     line.ThrowIfNotExpectedError();
                     throw new Exception(line.Payload.GetString() ?? "unknown error");
                 case "heartbeat":
+                    _logger.LogTrace("Heartbeat received.");
                     continue;
                 default:
                     throw new Exception($"Failed to handle unsupported line type '{line.Type}'.");