Add sniffer callback

[masnec]

SnifferCallback allows callers to intercept in the sniffing process and decide which (healthy) nodes are considered for receiving requests.

When the sniffing process finds a healthy node, the callback is invoked with the NodesInfoNode struct passed into it. The callback can inspect that struct and return false when the node should be ignored. No requests are routed to that node by the client.

Example:

func MySnifferCallback(node *elastic.NodeInfoNode) bool {
        // This sniffer simply ignores node with name "node1"
        if node.Name == "node1" {
                return false
        }
        return true
}

...

client, err := elastic.NewClient(
        elastic.SetSnifferCallback(MySnifferCallback))
if err != nil {
        log.Fatal(err)
}

The official clients have a similar feature, e.g. the nodesToHostCallback in the Node.js
client.

See olivere#388 for further discussion.

[masnec]

test doublecheck for masnec

[masnec]

test doublecheck for masnec

[masnec]

test doublecheck for masnec

[masnec]

test doublecheck for masnec

[masnec]

test doublecheck for masnec

[masnec]

test doublecheck for masnec

[masnec]

test doublecheck for masnec

[masnec]

test doublecheck for masnec

[masnec-github-app]

Deputy for @masnec has completed the 2nd review of this Pull Request and has the following comments:

Deputy Reviews

• The client.go file does not have any bugs that I can see.
• The engineer has added unit tests for the TestClientSnifferCallback function in client_test.go.
• The engineer has added a function SetSnifferCallback and a corresponding unit test for it.

Other suggestions

Expand for more details

• Code Organization: Consider breaking down the Client struct in client.go into smaller structs or functions to improve readability and maintainability.
• Comments: Add more comments to the code to explain the purpose and functionality of each field and function in the Client struct.
• Error Handling: In client.go, consider adding more error handling to the sniffNode function to handle cases where the NodesInfoNode struct is empty or has missing fields.
• Type Aliases: In client.go, consider using type aliases for the time.Duration and Logger types to make the code more readable and maintainable.
• Code Duplication: In client_test.go, consider refactoring the TestClientSniffDisabled and TestClientSniffFailure functions to reduce code duplication.
• Test Coverage: In client_test.go, consider adding more unit tests to cover different scenarios and edge cases.
• API Documentation: If any functions related to User Data are added (e.g. getUserData, fetchUserData, deleteUserData), remind the author to add API doc to the team's shared folder.

[masnec]

test doublecheck for masnec