Add token support for API calls

[geoffmore]

Previously, I opened #84 with a desire to support tokens in a similar manner manner to the other native authentication methods within elasticsearch. Although I wanted this as a feature before, I now consider this a bug. To my knowledge, all other authentication methods are implemented, so this would ensure consistency with the upstream product.

[karmi]

Hello, sorry for a long delay. We had a discussion about the token authentication internally, and at the moment it seems like it's not a good candidate for some higher-level exposure in the client API — there are various potential usability issues, like the tokens expiring, and so on.

I've added global HTTP request headers support in https://github.com/elastic/go-elasticsearch/commit/3f39d542adc013c69c7818092e6ac0d428e1b9a7, which should solve this problem in a rather unobtrusive way — and also allow setting other global custom headers, eg. Accept.

I've tested the token authentication locally, by first creating the token:

curl -k -X POST "https://elastic:elastic@localhost:9200/_security/oauth2/token?pretty" -H 'Content-Type: application/json' -d '{ "grant_type" : "client_credentials" }'

Then, I've run a script like this:

hdr := http.Header{}
hdr.Set("Authorization", "Bearer 24ToAx...")

es, _ := elasticsearch.NewClient(
		Logger: &estransport.ColorLogger{
			Output:             os.Stdout,
			EnableRequestBody:  true,
			EnableResponseBody: true,
		},

		Header: hdr,
	},
)

res, _ := es.Info()
log.Println(res)

Does this work for you?