PeriodicReader.Shutdown now applies the periodic reader's timeout by default

[dashpole]

From https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/metrics/sdk.md#shutdown,

Shutdown SHOULD complete or abort within some timeout. Shutdown MAY be implemented as a blocking API or an asynchronous API which notifies the caller via a callback or an event. OpenTelemetry SDK authors MAY decide if they want to make the shutdown timeout configurable.

We don't currently have a timeout, so this PR adds a timeout to meet the specification.

We could hard-code the timeout, but I think it makes the most sense to re-use the timeout set with WithTimeout(), since Shutdown essentially is a collectAndExport call, which the shutdown is intended for. We can always add a separate timeout for Shutdown, as long as it defaults to the general timeout.

Fixes #3663

This is also relevant for #3666. For MetricExporter.Export:

Export MUST NOT block indefinitely, there MUST be a reasonable upper limit after which the call must time out with an error result (Failure).

That implies we should set a timeout on contexts we pass to Export, which this PR does.

[codecov]

Codecov Report

Merging #4356 (2c41d82) into main (d8d3502) will decrease coverage by 0.1%.
The diff coverage is 100.0%.

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #4356     +/-   ##
=======================================
- Coverage   83.4%   83.4%   -0.1%     
=======================================
  Files        184     184             
  Lines      14374   14376      +2     
=======================================
- Hits       12001   11999      -2     
- Misses      2145    2149      +4     
  Partials     228     228             

Files Changed	Coverage Δ
sdk/metric/periodic_reader.go	84.1% <100.0%> (+0.1%)	⬆️

... and 2 files with indirect coverage changes

[MrAlias]

Can we update WithTimeout documentation to include this behavior?

[dashpole]

Can we update WithTimeout documentation to include this behavior?

Done