Ability to add custom metric attributes to http

[jurabek]

The current version of opentelemetry-instrumentation-http has a limitation where it removes attributes from outgoing-request and incoming-request metrics due to high cardinality issues. However, it would be useful to have the capability of adding other attributes to the HTTP metrics according to the user's requirements. For instance, if I want to filter and monitor specific downstream requests, adding http_target to the outgoing-request metric does not create high cardinality issues for me or other attributes e.g http_status_family: 2xx

Options would be by passing metricAttributes into applyCustomAttributesOnSpan

export interface HttpCustomAttributeFunction {
  (
    span: Span,
    metricAttributes: Attributes,
    request: ClientRequest | IncomingMessage,
    response: IncomingMessage | ServerResponse
  ): void;
}

cc: @hectorhdzg @legendecas for opinions and feedback 😊
Thank you 🙏