Add max_spans limit to InMemorySpanExporter to prevent unbounded memory growth

[shayannab]

Is your feature request related to a problem?

While building a local observability dashboard on top of the OpenTelemetry Python SDK, I noticed that InMemorySpanExporter stores all exported spans in an unbounded list with no way to cap the size.
InMemorySpanExporter stores all exported spans in an unbounded list (self._finished_spans). In a short-lived test this is fine, but in any long-running process — a dev server, a background worker, or a monitoring loop these spans accumulate indefinitely and memory grows without bound until the process crashes or gets killed.

Describe the solution you'd like

Add an optional max_spans: int parameter to init (defaulting to None for current unbounded behavior). When the limit is reached, either discard incoming spans or evict the oldest ones using a circular buffer (collections.deque(maxlen=max_spans)). The deque approach would be a one-line change to the existing implementation and would preserve the most recent spans automatically.

Describe alternatives you've considered

Manually calling clear() periodically — but this loses all span history rather than keeping the most recent ones.

Additional Context

This is particularly useful for developers using InMemorySpanExporter outside of pure unit tests, such as local observability dashboards or dev servers.

Would you like to implement a fix?

Yes

Tip

_{React with 👍 to help prioritize this issue. Please use comments to provide useful context, avoiding +1 or me too, to help us triage it. Learn more here.}