LangChain callback handler keeps runs that never end in memory #25510

createLangChainCallbackHandler() tracks each run in a map until its end or error callback fires. If LangChain never calls either, for example when a consumer abandons a stream, the entry and its span stay in the map forever. The auto-instrumentation path shares one handler for the whole process, so these add up. This predates #25364 (spanMap had it). Since #25364 each entry also holds the span that was active when the run started, which can keep a caller span outside the run's own tree alive.

Options: drop entries whose span has already ended when a new run starts, cap the map size, and store the active span's id instead of the span object.

Raised by @isaacs in #25364 (comment).

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