Enable OTel API instrumentation for generating custom spans (#1058)

### Problem
When using the Application Signals lambda layer for auto-instrumenting a
lambda function, we found that if we want to generate custom spans using
the OTel APIs, then these spans are generated with no-op context and are
not available in Application Signals.

### Steps to reproduce
- Create a Java lambda function (can use [this pre-build
app](https://github.com/awsdocs/aws-lambda-developer-guide/tree/main/sample-apps/blank-java))
with the following code
```java
package example;

import com.amazonaws.services.lambda.runtime.Context;
import com.amazonaws.services.lambda.runtime.LambdaLogger;
import com.amazonaws.services.lambda.runtime.RequestHandler;

import java.util.Map;

import software.amazon.awssdk.services.lambda.LambdaClient;
import software.amazon.awssdk.services.lambda.model.GetAccountSettingsResponse;
import software.amazon.awssdk.services.lambda.model.LambdaException;

import io.opentelemetry.api.GlobalOpenTelemetry;
import io.opentelemetry.api.trace.Span;
import io.opentelemetry.api.trace.Tracer;
import io.opentelemetry.api.trace.SpanKind;

// Handler value: example.Handler
public class Handler implements RequestHandler<Map<String,String>, String> {

    private static final LambdaClient lambdaClient = LambdaClient.builder().build();

    @OverRide
    public String handleRequest(Map<String,String> event, Context context) {

        LambdaLogger logger = context.getLogger();
        logger.log("Handler invoked");

        GetAccountSettingsResponse response = null;
        try {
            response = lambdaClient.getAccountSettings();
        } catch(LambdaException e) {
            logger.log(e.getMessage());
        }

        logger.log("=====1.");
        Span parentSpan = Span.current();
        logger.log(parentSpan.toString());

        if (GlobalOpenTelemetry.get() == null) {
          logger.log("----- GlobalOTelTracer is null");
        } else {
          logger.log("----- GlobalOTelTracer is initialized");
        }

        Tracer tracer = GlobalOpenTelemetry.getTracer("my-app");
        Span span = tracer.spanBuilder("rollTheDice")
                .setSpanKind(SpanKind.SERVER)
                .setParent(io.opentelemetry.context.Context.current().with(parentSpan))
                .startSpan();
        span.end();
        logger.log(span.toString());

        logger.log("=====2.");
        return response != null ? "Total code size for your account is " + response.accountLimit().totalCodeSize() + " bytes" : "Error";
    }
}
```
- Enable Application Signals from the Lambda function console.
- Invoke the function and observe the CW logs. You should see the
following lines where the two spans are both no-op
```
Handler invoked
--
=====1.
PropagatedSpan{ImmutableSpanContext{traceId=00000000000000000000000000000000, spanId=0000000000000000, traceFlags=00, traceState=ArrayBasedTraceState{entries=[]}, remote=false, valid=false}}
----- GlobalOTelTracer is initialized
PropagatedSpan{ImmutableSpanContext{traceId=00000000000000000000000000000000, spanId=0000000000000000, traceFlags=00, traceState=ArrayBasedTraceState{entries=[]}, remote=false, valid=false}}
=====2.
```

### Solution
- OpenTelemetry treats the [`opentelemetry-api` just like any other
library
instrumentation](https://github.com/open-telemetry/opentelemetry-java-instrumentation/tree/main/instrumentation/opentelemetry-api).
When instrumented, the methods (like `startSpan`) from the OpenTelemetry
API will produce spans that have proper context.
- In the Application Lambda layer, [only a handful of library
instrumentations are enabled by
default](https://github.com/aws-observability/aws-otel-java-instrumentation/blob/43bc6bcc795749e043aee5c64f794a525139d975/lambda-layer/otel-instrument#L38-L44).
This does not include the `opentelemetry-api` instrumentation.
- Adding the `OTEL_INSTRUMENTATION_OPENTELEMETRY_API_ENABLED=true`
environment variable will enable this instrumentation.

### Testing
- Once the `OTEL_INSTRUMENTATION_OPENTELEMETRY_API_ENABLED=true` was
set, the same code produced correct spans
```
Handler invoked
--
=====1.
ApplicationSpan{agentSpan=SdkSpan{traceId=67dc8edd575d3de05e89c56b19240ffd, spanId=d9039dec8378a88c, parentSpanContext=ImmutableSpanContext{traceId=67dc8edd575d3de05e89c56b19240ffd, spanId=796364ca3c8551b1, traceFlags=01, traceState=ArrayBasedTraceState{entries=[]}, remote=true, valid=true}, name=blank-java-function-Hs0Awjtfeqgx, kind=SERVER, attributes=AttributesMap{data={thread.name=main, cloud.resource_id=arn:aws:lambda:us-west-2:702258172533:function:blank-java-function-Hs0Awjtfeqgx, thread.id=1, cloud.account.id=702258172533, faas.invocation_id=4ca9914c-ba2b-4236-ba0d-8bc176593816}, capacity=128, totalAddedValues=5}, status=ImmutableStatusData{statusCode=UNSET, description=}, totalRecordedEvents=0, totalRecordedLinks=0, startEpochNanos=1742507745533600918, endEpochNanos=0}}
----- GlobalOTelTracer is initialized
ApplicationSpan{agentSpan=SdkSpan{traceId=67dc8edd575d3de05e89c56b19240ffd, spanId=62d682028539df0b, parentSpanContext=ImmutableSpanContext{traceId=67dc8edd575d3de05e89c56b19240ffd, spanId=d9039dec8378a88c, traceFlags=01, traceState=ArrayBasedTraceState{entries=[]}, remote=false, valid=true}, name=rollTheDice, kind=SERVER, attributes=AttributesMap{data={thread.name=main, thread.id=1, aws.local.operation=blank-java-function-Hs0Awjtfeqgx/FunctionHandler}, capacity=128, totalAddedValues=3}, status=ImmutableStatusData{statusCode=UNSET, description=}, totalRecordedEvents=0, totalRecordedLinks=0, startEpochNanos=1742507746224698439, endEpochNanos=1742507746224823765}}
=====2.
```


By submitting this pull request, I confirm that my contribution is made
under the terms of the Apache 2.0 license.

Author: srprash

lambda-layer/otel-instrument

@@ -37,6 +37,7 @@ fi
 
 # Enable default instrumentations
 export OTEL_INSTRUMENTATION_COMMON_DEFAULT_ENABLED=${OTEL_INSTRUMENTATION_COMMON_DEFAULT_ENABLED:-"false"}
+export OTEL_INSTRUMENTATION_OPENTELEMETRY_API_ENABLED=${OTEL_INSTRUMENTATION_OPENTELEMETRY_API_ENABLED:-"true"}
 export OTEL_INSTRUMENTATION_AWS_LAMBDA_ENABLED=${OTEL_INSTRUMENTATION_AWS_LAMBDA_ENABLED:-"true"}
 export OTEL_INSTRUMENTATION_AWS_SDK_ENABLED=${OTEL_INSTRUMENTATION_AWS_SDK_ENABLED:-"true"}
 export OTEL_INSTRUMENTATION_APACHE_HTTPCLIENT_ENABLED=${OTEL_INSTRUMENTATION_APACHE_HTTPCLIENT_ENABLED:-"true"}