Fixing performance regression in EventSource

[rcj1]

This PR is intended to close #117203 by fetching EventMetadata by reference instead of by value. Accessing the uninitialized TraceLoggingEventTypes field of EventMetadata structs will run a constructor; if the struct is accessed by value, this constructor runs repeatedly on WriteEventVarargs, in addition to the typical overhead of accessing by value vs by reference.

Below are performance benchmark test results on my local machine (Windows x64)

Microsoft.Extensions.Logging.EventSourceLogger

Method	HasSubscribers	Json	Mean	Error	StdDev	Median	Min	Max	Gen0	Allocated
NestedScopes_TwoMessages	False	False	66.65 ns	12.78 ns	14.72 ns	62.90 ns	50.07 ns	103.5 ns	0.0019	24 B
NestedScopes_TwoMessages	False	True	88.13 ns	34.40 ns	39.61 ns	75.93 ns	49.05 ns	158.7 ns	0.0018	24 B
NestedScopes_TwoMessages	True	False	1,865.06 ns	131.82 ns	151.80 ns	1,852.22 ns	1,673.65 ns	2,245.5 ns	0.4971	6256 B
NestedScopes_TwoMessages	True	True	2,101.99 ns	469.80 ns	541.02 ns	1,980.80 ns	1,648.20 ns	4,039.1 ns	0.4920	6256 B

System.Diagnostics.Perf_EventSource

Method	Mean	Error	StdDev	Median	Min	Max	Gen0	Allocated
Log	117.2 ns	3.90 ns	4.49 ns	116.7 ns	111.8 ns	127.0 ns	0.0232	296 B

System.Diagnostics.Tracing.Perf_EventListener

Method	Mean	Error	StdDev	Median	Min	Max	Gen0	Allocated
WriteEvent_NoParams	49.70 ns	2.437 ns	2.709 ns	49.86 ns	45.84 ns	55.84 ns	0.0057	72 B
WriteEvent_IntParams	69.31 ns	2.776 ns	3.197 ns	68.21 ns	64.94 ns	76.14 ns	0.0170	216 B
WriteEvent_StringParams	75.52 ns	2.703 ns	3.113 ns	75.14 ns	71.29 ns	82.14 ns	0.0197	248 B
WriteEvent_MixedParams	77.85 ns	2.863 ns	3.182 ns	76.61 ns	73.60 ns	84.66 ns	0.0176	224 B

Also closes dotnet/perf-autofiling-issues#58568

[Copilot]

Pull Request Overview

This PR improves performance by fetching EventMetadata by reference instead of by value to avoid repeated struct copies and constructor invocations.

• Replaced direct m_eventData[eventId] indexing with CollectionsMarshal.GetValueRefOrNullRef in exception handling and event serialization paths.
• Updated WriteEventWithRelatedActivityIdCore, WriteEventVarargs, SerializeEventArgs, LogEventArgsMismatches, and IsEnabledByDefault to use reference-based metadata access.

[dotnet-policy-service]

Tagging subscribers to this area: @tarekgh, @tommcdon, @steveisok, @pjanotti
See info in area-owners.md if you want to be subscribed.

[noahfalk]

LGTM

[steveisok]

Nice!