fix: Metrics throws exception when decorating non handler methods

[hjgraca]

Please provide the issue number

Issue number: #498

Summary

Even though Metrics decorator should be used exclusively to decorate the handler, utilities should not throw exceptions when used incorrectly.

Update Metrics constructor to not return and set all values.

Current Behaviour

Currently Metrics throws NullReferenceException when decorating non handler (no Lambda context) methods.

System.NullReferenceException: Object reference not set to an instance of an object.
   at AWS.Lambda.Powertools.Metrics.Metrics.AWS.Lambda.Powertools.Metrics.IMetrics.Flush(Boolean metricsOverflow) in /aws-lambda-powertools-dotnet/libraries/src/AWS.Lambda.Powertools.Metrics/Metrics.cs:line 204
   at AWS.Lambda.Powertools.Metrics.MetricsAspectHandler.OnExit(AspectEventArgs eventArgs) in /aws-lambda-powertools-dotnet/libraries/src/AWS.Lambda.Powertools.Metrics/Internal/MetricsAspectHandler.cs:line 71
   at AWS.Lambda.Powertools.Common.MethodAspectAttribute.WrapSync[T](Func`2 target, Object[] args, AspectEventArgs eventArgs) in /aws-lambda-powertools-dotnet/libraries/src/AWS.Lambda.Powertools.Common/Aspects/MethodAspectAttribute.cs:line 72
   at AWS.Lambda.Powertools.Common.UniversalWrapperAspect.Handle(Object instance, Type type, MethodBase method, Func`2 target, String name, Object[] args, Type returnType, Attribute[] triggers) in /aws-lambda-powertools-dotnet/libraries/src/AWS.Lambda.Powertools.Common/Aspects/UniversalWrapperAspect.cs:line 87
   at AWS.Lambda.Powertools.Metrics.Tests.Handlers.ExceptionFunctionHandler.__a$_around_MethodDecorated_100663324_w_0(Object[] )

Flush method has a null _context

void IMetrics.Flush(bool metricsOverflow)
    {
        if (_context.GetMetrics().Count == 0
            && _raiseOnEmptyMetrics)
            throw new SchemaValidationException(true);

        if (_context.IsSerializable)
        {
            var emfPayload = _context.Serialize();

            Console.WriteLine(emfPayload);

            _context.ClearMetrics();

            if (!metricsOverflow) _context.ClearNonDefaultDimensions();
        }
        else
        {
            if (!_captureColdStartEnabled)
                Console.WriteLine(
                    "##WARNING## Metrics and Metadata have not been specified. No data will be sent to Cloudwatch Metrics.");
        }
    }

Changes

Please provide a summary of what's being changed

Fix : always set the values, but only set instance if null.

    internal Metrics(IPowertoolsConfigurations powertoolsConfigurations, string nameSpace = null, string service = null,
        bool raiseOnEmptyMetrics = false, bool captureColdStartEnabled = false)
    {
        _instance ??= this; // this line updated

        _powertoolsConfigurations = powertoolsConfigurations;
        _raiseOnEmptyMetrics = raiseOnEmptyMetrics;
        _captureColdStartEnabled = captureColdStartEnabled;
        _context = InitializeContext(nameSpace, service, null);
        
        _powertoolsConfigurations.SetExecutionEnvironment(this);
    }

User experience

Please share what the user experience looks like before and after this change

Checklist

Please leave checklist items unchecked if they do not apply to your change.

• Meets tenets criteria
• I have performed a self-review of this change
• Changes have been tested
• Changes are documented
• PR title follows conventional commit semantics

Is this a breaking change?

RFC issue number:

Checklist:

• Migration process documented
• Implement warnings (if it can live side by side)

Acknowledgment

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[codecov-commenter]

Codecov Report

Attention: 17 lines in your changes are missing coverage. Please review.

Comparison is base (d8e60bf) 74.46% compared to head (95d300b) 74.79%.

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Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #499      +/-   ##
===========================================
+ Coverage    74.46%   74.79%   +0.33%     
===========================================
  Files           99       99              
  Lines         4041     4047       +6     
===========================================
+ Hits          3009     3027      +18     
+ Misses        1032     1020      -12     

Flag	Coverage Δ
unittests	74.79% <19.04%> (+0.33%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files	Coverage Δ
...aries/src/AWS.Lambda.Powertools.Metrics/Metrics.cs	81.63% <19.04%> (-4.19%)	⬇️

... and 4 files with indirect coverage changes

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[sonarqubecloud]

Kudos, SonarCloud Quality Gate passed!   

0 Bugs
0 Vulnerabilities
0 Security Hotspots
3 Code Smells

No Coverage information
0.0% Duplication

[amirkaws]

LGTM

[amirkaws]

remove this line