Review fixes for grizzly-2.3:javaagent

[otelbot]

Automated code review of instrumentation/grizzly-2.3/javaagent.

gpt-5.3-codex

Status	File	Category	Note
Fixed	DefaultFilterChainInstrumentation:33	Javaagent	Removed redundant isMethod() from transform() matcher chain.
Fixed	HttpHandlerInstrumentation:36	Javaagent	Removed redundant isMethod() from transform() matcher chain.
Fixed	HttpRequestHeadersGetter:15	Style	Converted enum singleton getter to class and replaced singleton usage with direct instance creation.
Fixed	HttpRequestHeadersGetter:24	Style	Added upstream SDK nullability contract handling (@Nullable + null guards) for TextMapGetter#get and getAll.
Fixed	GrizzlySingletons:23	Style	Replaced HttpRequestHeadersGetter.INSTANCE with new HttpRequestHeadersGetter().
Fixed	HttpServerFilterInstrumentation:53	Style	Replaced GrizzlyHttpResponseMutator.INSTANCE with new GrizzlyHttpResponseMutator() and converted mutator enum singleton to class.

Fixed: 6
Needs Manual Fix: 0

claude-sonnet-4.6

No issues found.