Add operationName variable for health_check_query

[kutysam]

Fixes #835

There is another issue that is slightly related #768 but the code is standalone for healthcheckquery, thus I am fixing this just for health check.

This is the payload for the data being sent by apollographql healthcheck and it is missing operationName

Why do we need this?

1. If you do a call with graphiql, you can see a operationName key being sent out too.
2. This operationName will be logged in metrics such as gql.operation.name in netflix dgs framework metrics https://netflix.github.io/dgs/advanced/instrumentation/#shared-tags
3. For us to filter out health check requests, we can use operationName to filter them out.

These images show the payload within graphiql

[apollo-cla]

@kutysam: Thank you for submitting a pull request! Before we can merge it, you'll need to sign the Apollo Contributor License Agreement here: https://contribute.apollographql.com/

[kutysam]

I've signed the agreement..
The ci is failing but I'm sure it is not due to my commit.

[trevor-scheer]

I've suggested we stick to the exact operation name with the double underscore convention.
Otherwise, LGTM! Can you please add an entry to gateway-js/CHANGELOG.md?

[trevor-scheer]

Suggested change

	export const HEALTH_CHECK_QUERY_OPERATION_NAME = 'ApolloServiceHealthCheck';
	export const HEALTH_CHECK_QUERY_OPERATION_NAME = '__ApolloServiceHealthCheck__';

[glasser]

Is this really necessary? See my comment: #835 (comment)

If we suddenly have a pressing need to go above and beyond the standard GraphQL-over-HTTP protocol and always include operationName even when there's just one operation, then this would just be the tip of the iceberg. I'm sure there are hundreds of places across Apollo open source projects where we would need to make a similar change.

[glasser]

Will fix #835 in #870.