refactor(instrumentation-http): fix eslint warnings

Fix the following eslint warnings:

```
/home/runner/work/opentelemetry-js/opentelemetry-js/experimental/packages/opentelemetry-instrumentation-http/src/http.ts
  508:17  warning  Forbidden non-null assertion              @typescript-eslint/no-non-null-assertion
  931:11  warning  Forbidden non-null assertion              @typescript-eslint/no-non-null-assertion
  1032:13  warning  Forbidden non-null assertion              @typescript-eslint/no-non-null-assertion
  1043:13  warning  Forbidden non-null assertion              @typescript-eslint/no-non-null-assertion
```

Because the expression we check is indirected through the a method
call (`getConfig()`), TypeScript cannot assume the value would be
the same across the two calls. By extracting the value and checking
for that, TypeScript can narrow the type correctly and we can avoid
the non-null assertion.

Ref open-telemetry#5365

experimental/packages/opentelemetry-instrumentation-http/src/http.ts

@@ -462,9 +462,7 @@ export class HttpInstrumentation extends InstrumentationBase<HttpInstrumentation
     oldMetricAttributes: Attributes,
     stableMetricAttributes: Attributes
   ): http.ClientRequest {
-    if (this.getConfig().requestHook) {
-      this._callRequestHook(span, request);
-    }
+    this._callRequestHook(span, request);
 
     /**
      * Determines if the request has errored or the response has ended/errored.
@@ -493,9 +491,7 @@ export class HttpInstrumentation extends InstrumentationBase<HttpInstrumentation
           getOutgoingRequestMetricAttributesOnResponse(responseAttributes)
         );
 
-        if (this.getConfig().responseHook) {
-          this._callResponseHook(span, response);
-        }
+        this._callResponseHook(span, response);
 
         this._headerCapture.client.captureRequestHeaders(span, header =>
           request.getHeader(header)
@@ -526,14 +522,11 @@ export class HttpInstrumentation extends InstrumentationBase<HttpInstrumentation
 
           span.setStatus(status);
 
-          if (this.getConfig().applyCustomAttributesOnSpan) {
+          const { applyCustomAttributesOnSpan } = this.getConfig();
+
+          if (applyCustomAttributesOnSpan) {
             safeExecuteInTheMiddle(
-              () =>
-                this.getConfig().applyCustomAttributesOnSpan!(
-                  span,
-                  request,
-                  response
-                ),
+              () => applyCustomAttributesOnSpan(span, request, response),
               () => {},
               true
             );
@@ -699,12 +692,8 @@ export class HttpInstrumentation extends InstrumentationBase<HttpInstrumentation
           context.bind(context.active(), request);
           context.bind(context.active(), response);
 
-          if (instrumentation.getConfig().requestHook) {
-            instrumentation._callRequestHook(span, request);
-          }
-          if (instrumentation.getConfig().responseHook) {
-            instrumentation._callResponseHook(span, response);
-          }
+          instrumentation._callRequestHook(span, request);
+          instrumentation._callResponseHook(span, response);
 
           instrumentation._headerCapture.server.captureRequestHeaders(
             span,
@@ -949,14 +938,11 @@ export class HttpInstrumentation extends InstrumentationBase<HttpInstrumentation
       span.updateName(`${request.method || 'GET'} ${route}`);
     }
 
-    if (this.getConfig().applyCustomAttributesOnSpan) {
+    const { applyCustomAttributesOnSpan } = this.getConfig();
+
+    if (applyCustomAttributesOnSpan) {
       safeExecuteInTheMiddle(
-        () =>
-          this.getConfig().applyCustomAttributesOnSpan!(
-            span,
-            request,
-            response
-          ),
+        () => applyCustomAttributesOnSpan(span, request, response),
         () => {},
         true
       );
@@ -1052,22 +1038,30 @@ export class HttpInstrumentation extends InstrumentationBase<HttpInstrumentation
     span: Span,
     response: http.IncomingMessage | http.ServerResponse
   ) {
-    safeExecuteInTheMiddle(
-      () => this.getConfig().responseHook!(span, response),
-      () => {},
-      true
-    );
+    const { responseHook } = this.getConfig();
+
+    if (responseHook) {
+      safeExecuteInTheMiddle(
+        () => responseHook(span, response),
+        () => {},
+        true
+      );
+    }
   }
 
   private _callRequestHook(
     span: Span,
     request: http.ClientRequest | http.IncomingMessage
   ) {
-    safeExecuteInTheMiddle(
-      () => this.getConfig().requestHook!(span, request),
-      () => {},
-      true
-    );
+    const { requestHook } = this.getConfig();
+
+    if (requestHook) {
+      safeExecuteInTheMiddle(
+        () => requestHook(span, request),
+        () => {},
+        true
+      );
+    }
   }
 
   private _callStartSpanHook(