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/[email protected]/trace/span.go:381 +0x25
go.opentelemetry.io/otel/sdk/trace.(*recordingSpan).End(0xc001358600, {0x0, 0x0, 0xc000ca2000?})
	go.opentelemetry.io/otel/[email protected]/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/[email protected]/transform/transform.go:650 +0xbb0
golang.org/x/text/cases.Caser.String(...)
	golang.org/x/[email protected]/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

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice.

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