Make Context.Sampled an int32

[bg451]

@bensigelman mentioned in a comment the idea of making (Context).Sampled an int32.

This would allow detailed sampling information to be stored, such as the sampling rate, whether the trace was forced sampled, etc.

[yurishkuro]

How would a sampling rate be stored, in some encoding? Why not report it out of band?

Ours is just a float number atm, although I can see it being stored as a power of 2 instead.

[bhs]

If I remember correctly (I might not!), the sampling rate in dapper was encoded as an "integer reciprocal"... i.e., 1024 for 1/1024 sampling. Some sort of fixed-point fraction would also work, I guess.

Reporting out-of-band: it's kind of a long story, but the short version is that it made things easier on the analysis size and it seemed like a small price to pay. (Certain statistical statements could be made locally rather than after trace assembly)

[yurishkuro]

In this case I would encode it as 1/(2^n) and only store n, which will need 4 or 5 bits at most (can still fit in byte).

But I have no objections to int32.