Support AppSignals metrics transport protocol configuration.

[zzhlogin]

Issue #, if available:

This PR is matching the aws-otel-java-instrumentation PR to support both http and grpc protocol according the environment variables.

Testing:
I test the change on EC2 instance using the CWAgent receiving the metrics and traces by running on two sets of environment variables:

1. http:

export OTEL_EXPORTER_OTLP_TRACES_PROTOCOL=http/protobuf
export OTEL_EXPORTER_OTLP_METRICS_PROTOCOL=http/protobuf
export OTEL_EXPORTER_OTLP_TRACES_ENDPOINT=http://localhost:4316/v1/traces
export OTEL_AWS_APP_SIGNALS_EXPORTER_ENDPOINT=http://localhost:4316/v1/metrics

2. grpc:

export OTEL_EXPORTER_OTLP_TRACES_PROTOCOL=grpc
export OTEL_EXPORTER_OTLP_METRICS_PROTOCOL=grpc
export OTEL_EXPORTER_OTLP_TRACES_ENDPOINT=http://localhost:4315
export OTEL_AWS_APP_SIGNALS_EXPORTER_ENDPOINT=http://localhost:4315

I has been confirmed that both the traces and metrics are received successfully, logs and service map shows up in the console:

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[wangzlei]

LGTM!