Improve StringBuilder usage

[BrennanConroy]

No description provided.

[BrennanConroy]

FYI @martincostello and @javiercn in regards to the conversation at https://github.com/dotnet/aspnetcore/pull/36368/files#r706039158

This makes use of Interpolated string handlers, and StringBuilder has a custom one which allows formatting the interpolated string directly into it's internal buffer instead of instantiating a string then copying the string.

Method	Mean	Error	StdDev	Gen0	Allocated
AppendToString	748.7 ns	12.33 ns	10.93 ns	0.1841	1448 B
AppendInterpolated	739.7 ns	9.29 ns	8.23 ns	0.0620	488 B

Benchmark code

[MemoryDiagnoser]
public class AppendBenchmark
{
    private byte[] _b = new byte[30];

    [GlobalSetup]
    public void Setup()
    {
        RandomNumberGenerator.Fill(_b);
    }

    [Benchmark]
    public string AppendToString()
    {
        var sb = new StringBuilder();
        foreach (var b in _b)
        {
            sb.Append(b.ToString("x2", CultureInfo.InvariantCulture));
        }
        return sb.ToString();
    }

    [Benchmark]
    public string AppendInterpolated()
    {
        var sb = new StringBuilder();
        foreach (var b in _b)
        {
            sb.Append(CultureInfo.InvariantCulture, $"{b:x2}");
        }
        return sb.ToString();
    }
}

[martincostello]

Nice 😎