converting eventsource metadata list to a dict

[rcj1]

Closes #99816

This pull request implements the request described in #99816 of reducing the memory usage of EventSource in the case of high EventIDs by using a dictionary of EventMetadata instead of an array. This change resulted in no visible slowdown of WriteEvent, and memory usage was brought down to consistent levels across high and low EventIDs that matches the preexisting memory usage levels in the case of low EventIDs.

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[Copilot]

Pull Request Overview

This PR replaces the fixed-size EventMetadata[] storage with a Dictionary<int, EventMetadata> to allow sparse and dynamic event ID mappings.

• Swaps all m_eventData array uses to Dictionary<int, EventMetadata> and updates access patterns to use CollectionsMarshal.
• Converts loops over array indices into dictionary key/value iterations and adapts initializers.
• Adds using System.Runtime.InteropServices for CollectionsMarshal APIs.

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 3 comments.

File	Description
src/libraries/System.Private.CoreLib/src/System/Diagnostics/Tracing/NativeRuntimeEventSource.cs	Switched m_eventData to a dictionary, updated bounds check and payload decoding to use CollectionsMarshal.
src/libraries/System.Private.CoreLib/src/System/Diagnostics/Tracing/EventSource.cs	Replaced array-based loops and initializers with dictionary equivalents; updated all indexing to use GetValueRefOrNullRef / GetValueRefOrAddDefault.

[rcj1]

Blocked by #117039

[noahfalk]

LGTM aside from a little bit of formatting stuff and a couple unneeded null checks.

Perf-wise I'd be a bit surprised if this change didn't incur at least a few % extra CPU cost in WriteEvent for a scenario with an active EventListener but I think it would be an OK tradeoff for the sparse event id memory benefits.