Performance improvements to CoberturaParser

[afscrome]

• Try and re-use already filtered XElements in CoberturaParser rather than re-filtering the xml document all over the place.
• Optimise some string.Concat in ProcessClass method which were showing up in hot paths in profiling

In local testing, this reduces parsing time from roughly 11.5secs to 9 seconds, as well as reducing memory allocations from 17GB to 2.2GB.

For reference below is the code I tested with

string[] coverageFiles = { /* Test files */ };
CoverageReportParser parser = new CoverageReportParser(1, 1, [],
    new DefaultFilter([]),
    new DefaultFilter([]),
    new DefaultFilter([]));

ReadOnlyCollection<string> collection = new(coverageFiles);
var stopwatch = Stopwatch.StartNew();
var results = parser.ParseFiles(collection);

var lines = results.Assemblies.Select(x => x.CoveredLines).Sum();

Console.WriteLine($"TIME: {stopwatch.Elapsed}, LINES: {lines}");

And the following show the GCStats results from PerfView

Before

• Runtime Version: V 8.0.824.36612
• CLR Startup Flags: 8388611
• Total CPU Time: 39,242 msec
• Total GC CPU Time: 3,510 msec
• Total Allocs : 17,090.138 MB
• Number of Heaps: 1
• GC CPU MSec/MB Alloc : 0.205 MSec/MB
• Total GC Pause: 4,301.4 msec
• % Time paused for Garbage Collection: 36.0%
• % CPU Time spent Garbage Collecting: 8.9%
• Max GC Heap Size: 740.916 MB
• Peak Process Working Set: 808.591 MB
• Peak Virtual Memory Usage: 2,480,934.539 MB

After

• Runtime Version: V 8.0.824.36612
• CLR Startup Flags: 8388611
• Total CPU Time: 19,845 msec
• Total GC CPU Time: 2,029 msec
• Total Allocs : 2,204.929 MB
• Number of Heaps: 1
• GC CPU MSec/MB Alloc : 0.920 MSec/MB
• Total GC Pause: 2,350.1 msec
• % Time paused for Garbage Collection: 29.2%
• % CPU Time spent Garbage Collecting: 10.2%
• Max GC Heap Size: 732.562 MB
• Peak Process Working Set: 796.983 MB
• Peak Virtual Memory Usage: 2,480,932.901 MB

[danielpalme]

Thank you! I will review and merge your PR as soon as possible.