Removed allocations for timestamp

[epeshk]

Similar to https://github.com/serilog/serilog/pull/2094

Removed allocations from DateTimeOffset.ToString by using ISpanFormattable.TryFormat API

[MemoryDiagnoser]
[SimpleJob(RuntimeMoniker.Net80)]
[SimpleJob(RuntimeMoniker.Net90)]
public class ExpressionBenchmark
{
  private TextWriter nullWriter = TextWriter.Null;
  private ExpressionTemplate expressionFormatter = new ExpressionTemplate("[{@t:HH:mm:ss.fff} {@l:u3} ({SourceContext})] {@m}\n");
  
  private readonly LogEvent logEvent = new LogEvent(DateTimeOffset.Now, LogEventLevel.Information, null, new MessageTemplateParser().Parse("Something happens in method {Method} with parameter {Param}, context: {Context}"), new LogEventProperty[
  ]
  {
    new LogEventProperty("Method", new ScalarValue("ProcessMessages")),
    new LogEventProperty("Param", new ScalarValue(12345678)),
    new LogEventProperty("Context", new ScalarValue("DummyContext"))
  });

  [Benchmark]
  public void WriteEvent() => expressionFormatter.Format(logEvent, nullWriter);
}

Before

Method	Job	Runtime	Mean	Error	StdDev	Gen0	Allocated
WriteEvent	.NET 8.0	.NET 8.0	222.9 ns	1.11 ns	0.86 ns	0.0210	176 B
WriteEvent	.NET 9.0	.NET 9.0	180.3 ns	1.52 ns	1.42 ns	0.0210	176 B

After

Method	Job	Runtime	Mean	Error	StdDev	Gen0	Allocated
WriteEvent	.NET 8.0	.NET 8.0	225.5 ns	1.52 ns	1.34 ns	0.0086	72 B
WriteEvent	.NET 9.0	.NET 9.0	163.0 ns	1.73 ns	1.54 ns	0.0086	72 B

[nblumhardt]

Hi @epeshk, thanks for sending this, looks like a fantastic improvement :+1:

DateTime(Offset) format strings don't have any natural maximum length in .NET, so the following kind of thing is valid:

Console.WriteLine("{0:----------------------------------------------}", DateTimeOffset.Now);

I think it's fine to apply a limitation like "format strings can't result in more than 36 chars" but perhaps this could be documented using a constant and a unit test, so that we can be sure if a format string ends up producing output longer than what we'd expect, the results are reasonable?

Definitely in favor of this change otherwise 👍

[nblumhardt]

Reading the code again, I see that we should fall through and still create correct output in the "too long" case - think it just needs a test :-)