ESQL: Add documents_found and values_loaded (elastic#125631)

This adds `documents_found` and `values_loaded` to the to the ESQL response:
```json
{
  "took" : 194,
  "is_partial" : false,
  "documents_found" : 100000,
  "values_loaded" : 200000,
  "columns" : [
    { "name" : "a", "type" : "long" },
    { "name" : "b", "type" : "long" }
  ],
  "values" : [[10, 1]]
}
```

These are cheap enough to collect that we can do it for every query and
return it with every response. It's small, but it still gives you a
reasonable sense of how much work Elasticsearch had to go through to
perform the query.

I've also added these two fields to the driver profile and task status:
```json
    "drivers" : [
      {
        "description" : "data",
        "cluster_name" : "runTask",
        "node_name" : "runTask-0",
        "start_millis" : 1742923173077,
        "stop_millis" : 1742923173087,
        "took_nanos" : 9557014,
        "cpu_nanos" : 9091340,
        "documents_found" : 5,   <---- THESE
        "values_loaded" : 15,    <---- THESE
        "iterations" : 6,
...
```

These are at a high level and should be easy to reason about. We'd like to
extract this into a "show me how difficult this running query is" API one
day. But today, just plumbing it into the debugging output is good.

Any `Operator` can claim to "find documents" or "load values" by overriding
a method on its `Operator.Status` implementation:
```java
/**
 * The number of documents found by this operator. Most operators
 * don't find documents and will return {@code 0} here.
 */
default long documentsFound() {
    return 0;
}

/**
 * The number of values loaded by this operator. Most operators
 * don't load values and will return {@code 0} here.
 */
default long valuesLoaded() {
    return 0;
}
```

In this PR all of the `LuceneOperator`s declare that each `position` they
emit is a "document found" and the `ValuesSourceValuesSourceReaderOperator`
says each value it makes is a "value loaded". That's pretty pretty much
true. The `LuceneCountOperator` and `LuceneMinMaxOperator` sort of pretend
that the count/min/max that they emit is a "document" - but that's good
enough to give you a sense of what's going on. It's *like* document.

Author: nik9000

docs/changelog/125631.yaml

@@ -0,0 +1,5 @@
+pr: 125631
+summary: Add `documents_found` and `values_loaded`
+area: ES|QL
+type: enhancement
+issues: []

docs/reference/esql/functions/description/knn.asciidoc

@@ -251,6 +251,7 @@ static TransportVersion def(int id) {
     public static final TransportVersion RANDOM_SAMPLER_QUERY_BUILDER_8_19 = def(8_841_0_56);
     public static final TransportVersion ML_INFERENCE_SAGEMAKER_ELASTIC_8_19 = def(8_841_0_57);
     public static final TransportVersion SPARSE_VECTOR_FIELD_PRUNING_OPTIONS_8_19 = def(8_841_0_58);
+    public static final TransportVersion ESQL_DOCUMENTS_FOUND_AND_VALUES_LOADED_8_19 = def(8_841_0_59); // NOCOMMIT open PR in main first
 
     /*
      * STOP! READ THIS FIRST! No, really,