Allow users to hide Gen X columns

[adamsitnik]

Some of our users, are not interested in the Gen 0/1/2 columns, all they want to see is the Allocated column.

Example:

public class AccurateAllocations
{
    [Benchmark] public byte[] EightBytesArray() => new byte[8];
    [Benchmark] public byte[] SixtyFourBytesArray() => new byte[64];
    [Benchmark] public Task<int> AllocateTask() => Task.FromResult(default(int));
}

Before:

Method	Mean	Error	StdDev	Gen 0	Gen 1	Gen 2	Allocated
EightBytesArray	4.245 ns	0.0804 ns	0.0671 ns	0.0041	-	-	32 B
SixtyFourBytesArray	8.160 ns	0.1413 ns	0.1180 ns	0.0112	-	-	88 B
AllocateTask	7.561 ns	0.1613 ns	0.1430 ns	0.0092	-	-	72 B

First commit (don't display Gen X collumn if there was no collections in Gen X):

Method	Mean	Error	StdDev	Gen 0	Allocated
EightBytesArray	4.184 ns	0.0623 ns	0.0552 ns	0.0041	32 B
SixtyFourBytesArray	8.255 ns	0.2031 ns	0.1801 ns	0.0112	88 B
AllocateTask	7.996 ns	0.1210 ns	0.1073 ns	0.0092	72 B

[MemoryDiagnoser(displayGenColumns: false)]

Method	Mean	Error	StdDev	Allocated
EightBytesArray	4.546 ns	0.1317 ns	0.1232 ns	32 B
SixtyFourBytesArray	8.713 ns	0.2251 ns	0.1995 ns	88 B
AllocateTask	8.324 ns	0.1991 ns	0.2213 ns	72 B

cc @stephentoub

[AndreyAkinshin]

It seems that we have some problems with the column order. Let's consider the following benchmark class:

[MemoryDiagnoser]
public class ColumnOrder
{
    [Benchmark] public byte[] A() => new byte[8];

    [Benchmark]
    public void B()
    {
        byte[] b = new byte[8];
        GC.Collect();
    }
}

It gives a summary table like this one:

| Method |          Mean |          Error |        StdDev |     Gen 0 | Allocated |     Gen 1 |     Gen 2 |
|------- |--------------:|---------------:|--------------:|----------:|----------:|----------:|----------:|
|      A |      2.720 ns |      0.6458 ns |     0.0354 ns |    0.0077 |      32 B |         - |         - |
|      B | 64,272.705 ns | 28,250.1984 ns | 1,548.4887 ns | 1000.0000 |      32 B | 1000.0000 | 1000.0000 |

As you can see, now we have the Allocated column between Gen 0 and Gen 1.

[adamsitnik]

@AndreyAkinshin great catch! I've fixed that in 51d39f0

[AndreyAkinshin]

@adamsitnik now it works great, thanks!