Use Volatile instead of Interlocked where appropriate

[pentp]

Volatile reads/writes are atomic and have acquire/release semantics, but are for the most part as fast as regular reads/writes. Any interlocked operation is at least 30-40 CPU cycles and needs exclusive cache line ownership, which is especially bad for reads.

Split off from #6048.

[cijothomas]

curious, if this shows improvement in the metric stress tests?

[utpilla]

We need to consider the memory ordering guarantees of Volatile.Write. With Interlocked methods, the read/writes would not be moved before or after a given Interlocked method.

With volatile writes, read/writes that happen after a given Volatile.Write method can be moved before that Volatile.Write method. We need to evaluate if that affects the correctness of our code. There are some write operations that we do after releasing the locks (for exemplar and MetricPoint updates):

• Call OnCollected for Exemplars which resets the internal measurement state
• Update MetricStatus to CollectPending when updating MetricPoints

[pentp]

I went over all uses of Interlocked and also checked the code flow after these lock releases. I now found a few places where the current code incorrectly relies on the preceding interlocked operation for memory ordering, for example in case of MetricPoint.UpdateWithExemplar the order of operations is currently:

Interlocked.Exchange(ref this.runningValue.AsLong, number); // full fence
this.UpdateExemplar(number, tags, offerExemplar); // could run arbitrary lock-free code, though in practice uses locks
this.MetricPointStatus = MetricPointStatus.CollectPending; // no memory ordering guarantees, could become observable before exemplar updates

With this PR:

Volatile.Write(ref this.runningValue.AsLong, number); // release
this.UpdateExemplar(number, tags, offerExemplar);
Volatile.Write(ref this.status, (byte)MetricPointStatus.CollectPending); // release, guarantees all exemplar updates become observable before

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 86.39%. Comparing base (cd31c57) to head (f1645d1).
Report is 7 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6051      +/-   ##
==========================================
- Coverage   86.39%   86.39%   -0.01%     
==========================================
  Files         257      257              
  Lines       11690    11668      -22     
==========================================
- Hits        10100    10080      -20     
+ Misses       1590     1588       -2     

Files with missing lines	Coverage Δ
...Protocol/PersistentStorage/DirectorySizeTracker.cs	45.83% <ø> (+3.52%)	⬆️
src/OpenTelemetry/Internal/InterlockedHelper.cs	100.00% <ø> (ø)
src/OpenTelemetry/Metrics/Exemplar/Exemplar.cs	86.66% <ø> (-0.44%)	⬇️
...c/OpenTelemetry/Metrics/MetricPoint/MetricPoint.cs	94.08% <ø> (-0.16%)	⬇️
...trics/MetricPoint/MetricPointOptionalComponents.cs	100.00% <ø> (ø)

[pentp]

Using auto-layout together with the field type change for the two enums below reduces the struct size from 72 bytes to 64 bytes.