sdk/metric: apply default cardinality limit of 2000

[pellared]

Per https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/metrics/sdk.md#cardinality-limits:

If none of the previous values are defined, the default value of 2000 SHOULD be used.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 82.6%. Comparing base (63ee0c5) to head (b7d787b).

Additional details and impacted files

@@          Coverage Diff          @@
##            main   #8247   +/-   ##
=====================================
  Coverage   82.6%   82.6%           
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  Files        309     309           
  Lines      24621   24621           
=====================================
  Hits       20356   20356           
- Misses      3888    3889    +1     
+ Partials     377     376    -1     

Files with missing lines	Coverage Δ
sdk/metric/config.go	95.2% <ø> (ø)

... and 2 files with indirect coverage changes

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[Copilot]

Pull request overview

Applies the Metrics SDK specification-recommended default cardinality limit (2000) in go.opentelemetry.io/otel/sdk/metric, updating tests, docs, and changelog to reflect the new default and overflow behavior.

Changes:

• Set the SDK default cardinality limit to 2000 (previously unlimited by default).
• Update cardinality-limit tests to validate overflow aggregation behavior.
• Refresh package docs, examples, and changelog to reflect the new default and migration guidance.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
sdk/metric/config.go	Changes the default cardinality limit constant to 2000 and updates option docs.
sdk/metric/provider_test.go	Adjusts cardinality limit tests for the new default and checks overflow datapoints.
sdk/metric/config_test.go	Extends env-var parsing tests (including 0 to disable the limit).
sdk/metric/doc.go	Updates package documentation to state the new default and describe overflow behavior.
sdk/metric/example_test.go	Removes redundant explicit WithCardinalityLimit(2000) since it’s now default.
CHANGELOG.md	Documents the default change and how to preserve unlimited cardinality.

[dmathieu]

I remember us making an explicit decision not to change that, as it was a breaking change.
I don't mind revisiting that decision, just pointing that out.

[dashpole]

I was initially opposed to doing this, but its becoming more of a wart with all of the CVE reports and with users expecting consistency with declarative config. I think as long as we keep the env var around for a while this is the best path forward.