Add context to periodic reader

[alecsammon]

In our usage of this we make use of slog context methods - i.e. slog.InfoContext

As the periodic reader uses a context.Background() this is causing issues with our logging as we are missing required data from the context.

I think that ideally NewPeriodicReader would take a context.Context as it's first parameter - i.e.
func NewPeriodicReader(ctx context.Context, exporter Exporter, options ...PeriodicReaderOption) *PeriodicReader {
However this would be a breaking change which may be undesirable.

There are few of option I believe.

1. Add a functional option to add a context. This is probably the cleanest option at the moment, but does break the standard go convention of not including contexts in structs.
2. Add a new method of NewPeriodicReaderContext that receives the context - this would be the least code change
3. Make a breaking change to the NewPeriodicReader

This PR shows the code required for option 1.
If option 2 of NewPeriodicReaderContext is preferred then I'm happy to create that PR instead.

Thanks!

[dmathieu]

A functional option is IMHO the best way indeed.
In this PR, you're setting the context in every test (though it's not necessary, so setting it is a bit of clutter). But nothing actually tests that the option actually uses the passed context.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 83.0%. Comparing base (4458a00) to head (ff69e1b).
Report is 5 commits behind head on main.

Additional details and impacted files

@@          Coverage Diff          @@
##            main   #6936   +/-   ##
=====================================
  Coverage   83.0%   83.0%           
=====================================
  Files        261     261           
  Lines      24256   24262    +6     
=====================================
+ Hits       20143   20148    +5     
- Misses      3738    3739    +1     
  Partials     375     375           

Files with missing lines	Coverage Δ
sdk/metric/periodic_reader.go	84.4% <100.0%> (-0.6%)	⬇️

... and 1 file with indirect coverage changes

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[alecsammon]

Thanks @dmathieu

Agree the tests on this PR are not good enough! I wanted to get clarity on way forward before investing too much time.

I'll get the PR ready with tests and changelog.