perf(logs): avoid string allocation when no parameters are passed

[Flash0ver]

Closes #5118.

Avoid a string allocation when there are no parameters that the template string should be formatted with.
Also, ensure that the Parameters of type ImmutableArray`1 are never the default-struct, so that we can avoid ImmutableArray.IsDefault checks afterwards.

@bitsandfoxes, we talked about this optimization quite a while back. It's mainly for this code path:

• https://github.com/getsentry/sentry-unity/blob/e2f2db400d776b26108759688003b8a120094538/src/Sentry.Unity/Integrations/UnityApplicationLoggingIntegration.cs#L158

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Semver Impact of This PR

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• perf(logs): avoid string allocation when no parameters are passed by Flash0ver in #4697

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Codecov Report

❌ Patch coverage is 93.33333% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 74.01%. Comparing base (39ea4d8) to head (ab926d9).

Files with missing lines	Patch %	Lines
...c/Sentry/Internal/DefaultSentryStructuredLogger.cs	87.50%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4697      +/-   ##
==========================================
- Coverage   74.12%   74.01%   -0.12%     
==========================================
  Files         499      499              
  Lines       18067    18071       +4     
  Branches     3520     3518       -2     
==========================================
- Hits        13392    13375      -17     
- Misses       3813     3838      +25     
+ Partials      862      858       -4     

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[Flash0ver]

note: perf comparison

Before this change:

Method	Mean	Error	StdDev	Median	Gen0	Allocated
LogWithoutParameters	122.2 ns	2.31 ns	5.87 ns	120.5 ns	0.0696	584 B
LogWithParameters	242.6 ns	0.79 ns	0.74 ns	242.5 ns	0.1087	912 B

After this change:

Method	Mean	Error	StdDev	Gen0	Gen1	Allocated
LogWithoutParameters	102.3 ns	1.28 ns	1.19 ns	0.0640	0.0001	536 B
LogWithParameters	248.5 ns	4.86 ns	5.40 ns	0.1087	-	912 B

Result:

• A tiny pessimization in time (IMO neglectable) with params/args
  • one more jump and assignment
• A notable optimization (a bit of time and 48 bytes in space) without params/args
  • no invocation of System.String.Format

[Flash0ver]

question: Breaking Change?

This improvement is - technically speaking - also a behavioral (breaking?) change, when Logging a plain message without positional arguments, observable by user code through the SetBeforeSendLog callback:

options.SetBeforeSendLog(static (SentryLog log) =>
{
    _ = (log.Message, log.Template, log.Parameters);
    return log;
});

No changes to the resulting serialized payload, though - we have a check in place that does not serialize a sentry.message.template when there is no sentry.message.parameter ... but there are user-code observable changes to the SentryLog.Template and SentryLog.Parameters properties.

---

when Logging without positional Parameters: e.g. SentrySdk.Logger.LogError("Without parameters");

• Before
  • log.Template == log.Message
  • log.Parameters.IsDefault == true
• After
  • log.Template == null
  • log.Parameters.IsDefault == false
• Justification
  • SentryLog.Template was already declared nullable (string?)
    • no contractual change
    • and this now also mirrors what is actually serialized for the payload
  • the ImmutableArray`1 of Parameters may no longer be default
    • when an ImmutableArray`1 is default, invoking any instance member other than IsDefault or IsDefaultOrEmpty will throw a NullReferenceException, because the underlying Array is null
    • this makes user-callbacks "safer"
    • but also the down-stream sentry-unity SDK

Another benefit of ImmutableArray<KeyValuePair<string, object>> Parameters no longer being default when logging without positional arguments is that we can omit IsDefault and IsDefaultOrEmpty checks in the ISentryJsonSerializable.WriteTo serialization logic, which makes it slightly faster, too.

---

when Logging with positional Parameters: e.g. SentrySdk.Logger.LogError("With parameters {0} {1} {2}", 1, 2, 3);

• no observable changes

---

See also https://develop.sentry.dev/sdk/telemetry/logs/#default-attributes.

[Flash0ver]

and a follow-up question:

Shall we note this in the CHANGELOG?

[Flash0ver]

note: this ensures that the ImmutableArray`1 of Parameters is never default

In sentry-dotnet, there is currently no code path that would require this, as we always set the instance to at least "Empty".

But in the down-stream sentry-unity, there actually is a code path where it may be default.

• see https://github.com/getsentry/sentry-unity/blob/e2f2db400d776b26108759688003b8a120094538/src/Sentry.Unity/Integrations/UnityApplicationLoggingIntegration.cs#L158-L159
• this is also the main scenario this optimization is for
• as in sentry-dotnet, we assume most usages via ILogger and other Logging-Integrations

With this assignment, we ensure that it's never default, and both us and users can omit IsDefault/IsDefaultOrEmpty checks.

[Flash0ver]

note: omit default checks

Since we ensured in the internal .ctor, as well as the init-only property, that the ImmutableArray is never default, and with that the underlying array never null, we can omit further IsDefault and IsDefaultOrEmpty checks.

[Flash0ver]

note: I wanted to keep the tests for both Logs and Metrics in sync