Add potential metric capitalization panic recovery for ApplicationSignals processor

[jefchien]

Description of the issue

There have been cases of the agent panicking from the capitalization of metrics in the Application Signals processor.

panic serving [::1]:64029: runtime error: slice bounds out of range [22:21]
goroutine 4539 [running]:
net/http.(*conn).serve.func1()
	net/http/server.go:1903 +0xbe
panic({0x105dc4ca0?, 0xc0011df338?})
	runtime/panic.go:770 +0x132
go.opentelemetry.io/otel/sdk/trace.(*recordingSpan).End.deferwrap1()
	go.opentelemetry.io/otel/sdk@v1.27.0/trace/span.go:381 +0x25
go.opentelemetry.io/otel/sdk/trace.(*recordingSpan).End(0xc001358600, {0x0, 0x0, 0xc000ca2000?})
	go.opentelemetry.io/otel/sdk@v1.27.0/trace/span.go:413 +0x9fc
panic({0x105dc4ca0?, 0xc0011df338?})
	runtime/panic.go:770 +0x132
golang.org/x/text/transform.String({0x14f6de010, 0xc0008b23c0}, {0xc0011de240, 0x15})
	golang.org/x/text@v0.16.0/transform/transform.go:650 +0xbb0
golang.org/x/text/cases.Caser.String(...)
	golang.org/x/text@v0.16.0/cases/cases.go:51
github.com/aws/amazon-cloudwatch-agent/plugins/processors/awsapplicationsignals.(*awsapplicationsignalsprocessor).processMetrics(0xc0009a02d0, {0x1061bf3e8, 0xc000e78540}, {0xc000140480?, 0xc00073a844?})
	github.com/aws/amazon-cloudwatch-agent/plugins/processors/awsapplicationsignals/processor.go:140 +0x1a5

It's still unclear why the metric name capitalization is causing a panic.

Description of changes

• Adds panic recovery, logging, and dropping of the metric if the processor is unable to capitalize the metric.
• Made the cases.Caser per instance instead of shared within the package to prevent issues with shared state.

https://pkg.go.dev/golang.org/x/text/cases#Caser

// A Caser may be stateful and should therefore not be shared between
// goroutines.

License

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Tests

Added unit tests.

Requirements

Before commit the code, please do the following steps.

1. Run make fmt and make fmt-sh
2. Run make lint