Added stats for phase results processors at search pipeline level

[martin-gaievski]

Description

Adding stats for search phase result processor type. Today we do not collect any metric for this processor type, while for request and response processor types those metrics are collected and returned with the node level stats response.

Sample request, no changes there

GET /_nodes/stats/search_pipeline

sample response, section phase_results_processors is added with this PR:

        "XzOye699Q-SnVrFcJ-b8Tg": {
            "timestamp": 1741224352625,
            "name": "integTest-0",
            "transport_address": "127.0.0.1:9300",
            "host": "127.0.0.1",
            "ip": "127.0.0.1:9300",
            "roles": [
                "cluster_manager",
                "data",
                "ingest",
                "remote_cluster_client"
            ],
            "attributes": {
                "testattr": "test",
                "shard_indexing_pressure_enabled": "true"
            },
            "search_pipeline": {
                "total_request": {
                    "count": 240,
                    "time_in_millis": 547,
                    "current": 0,
                    "failed": 0
                },
                "total_response": {
                    "count": 0,
                    "time_in_millis": 0,
                    "current": 0,
                    "failed": 0
                },
                "pipelines": {
                    "nlp-min-max-arithmetic-search-pipeline": {
                        "request": {
                            "count": 240,
                            "time_in_millis": 547,
                            "current": 0,
                            "failed": 0
                        },
                        "response": {
                            "count": 0,
                            "time_in_millis": 0,
                            "current": 0,
                            "failed": 0
                        },
                        "request_processors": [],
                        "response_processors": [],
                        "phase_results_processors": [
                            {
                                "normalization-processor": {
                                    "type": "normalization-processor",
                                    "stats": {
                                        "count": 240,
                                        "time_in_millis": 545,
                                        "current": 0,
                                        "failed": 0
                                    }
                                }
                            }
                        ]
                    }
                }
            }
        }, 

Related Issues

#17558

Check List

• Functionality includes testing.
• API changes companion pull request created, if applicable.
• Public documentation issue/PR created, if applicable.

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