[bug] fix MeterAdapter System.FormatException: Input string was not in a co…

[cmgt]

It's probably just a typo. But the problem is blocking.
Here prometheus-net-dotpulsar is an example to reproduce the error.

When you create a custom Meter specifying a unit, such as this:

m = new Meter("prometheus-dotpulsar", "1.0.0");
_ = m.CreateObservableGauge("dotpulsar.client.count", Inc, "{clients}", "Number of clients");

an unhandled exception occurs in the TranslateInstrumentDescriptionToPrometheusHelp method of the MeterAdapter class

prometheus-net/Prometheus/MeterAdapter.cs

Line 267 in 43b8625

sb.AppendFormat($"({instrument.Unit}) ");

System.FormatException: Input string was not in a correct format.
   at System.ThrowHelper.ThrowFormatInvalidString()
Exception thrown: 'System.FormatException' in System.Private.CoreLib.dll
System.FormatException: Input string was not in a correct format.
   at System.Text.StringBuilder.AppendFormatHelper(IFormatProvider provider, String format, ReadOnlySpan`1 args)
   at System.Text.StringBuilder.AppendFormat(String format, Object[] args)
   at Prometheus.MeterAdapter.TranslateInstrumentDescriptionToPrometheusHelp(Instrument instrument)
   at Prometheus.MeterAdapter.OnInstrumentPublished(Instrument instrument, MeterListener listener)
   at System.Diagnostics.Metrics.MeterListener.Start()
   at Prometheus.MeterAdapter..ctor(MeterAdapterOptions options)
   at Prometheus.MeterAdapter.StartListening(MeterAdapterOptions options)
   at Prometheus.SuppressDefaultMetricOptions.Configure(CollectorRegistry registry, ConfigurationCallbacks configurationCallbacks)
   at Prometheus.Metrics.<>c__DisplayClass23_0.<SuppressDefaultMetrics>b__0()
   at Prometheus.CollectorRegistry.CollectAndSerializeAsync(IMetricsSerializer serializer, CancellationToken cancel)
   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()

[sandersaares]

Good catch, thanks!