Avoid using interfaces/oneof like style for optional fields

[bogdandrutu]

This changes how to handle optional primitive fields, instead of using a "oneof" like style, define a metadata for each message that for the moment only contains information if the optional fields are present of not.

Alternative considered to use a pointer to the value instead of "any" that would reduce the size of the field from 16B to 8B but would keep an extra allocation.

Before:

BenchmarkMarshalAndUnmarshal/empty
BenchmarkMarshalAndUnmarshal/empty-12         	16115462	        73.64 ns/op	     224 B/op	       1 allocs/op
BenchmarkMarshalAndUnmarshal/all
BenchmarkMarshalAndUnmarshal/all-12           	 1844172	       641.0 ns/op	     800 B/op	      21 allocs/op

After:

BenchmarkMarshalAndUnmarshal/empty
BenchmarkMarshalAndUnmarshal/empty-12         	18948967	        62.91 ns/op	     208 B/op	       1 allocs/op
BenchmarkMarshalAndUnmarshal/all
BenchmarkMarshalAndUnmarshal/all-12           	 1914825	       624.3 ns/op	     760 B/op	      18 allocs/op

Bechmark code:

func BenchmarkMarshalAndUnmarshal(b *testing.B) {
	tests := map[string]*ExponentialHistogramDataPoint{
		"empty": NewExponentialHistogramDataPoint(),
		"all": GenTestExponentialHistogramDataPoint(),
	}
	for name, src := range tests {
		b.Run(name, func(b *testing.B) {
			buf := make([]byte, src.SizeProto())
			b.ReportAllocs()
			b.ResetTimer()
			for b.Loop() {
				gotSize := src.MarshalProto(buf)
				if len(buf) != gotSize {
					b.Fatalf("size mismatch: %d vs %d", len(buf), gotSize)
				}

				dest := NewExponentialHistogramDataPoint()
				if err := dest.UnmarshalProto(buf); err != nil {
					b.Fatal(err)
				}
			}
		})
	}
}

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.09%. Comparing base (a9aaa99) to head (41bbc46).
⚠️ Report is 4 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #14333      +/-   ##
==========================================
- Coverage   92.13%   92.09%   -0.05%     
==========================================
  Files         668      668              
  Lines       41529    41361     -168     
==========================================
- Hits        38263    38090     -173     
- Misses       2230     2231       +1     
- Partials     1036     1040       +4     

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[dmitryax]

LGTM 👍