Add search document count metric

[shivamg2017]

Description

Add search document count metric
Metric to fetch doc count returned by search query.

Relevant Opensearch PR to update SearchRequestContext : opensearch-project/OpenSearch#21037

metric: ImmutableMetricData{resource=Resource{schemaUrl=null, attributes={service.name="OpenSearch"}}, instrumentationScopeInfo=InstrumentationScopeInfo{name=org.opensearch.telemetry, version=null, schemaUrl=null, attributes={}}, name=search.query.doc_count.histogram, description=Histogram for the number of documents returned per search query, unit=1, type=EXPONENTIAL_HISTOGRAM, data=ImmutableExponentialHistogramData{aggregationTemporality=CUMULATIVE, points=[ImmutableExponentialHistogramPointData{getStartEpochNanos=1774810013641740000, getEpochNanos=1774810493652552000, getAttributes={}, getScale=20, getSum=12.0, getCount=4, getZeroCount=0, hasMin=true, getMin=3.0, hasMax=true, getMax=3.0, getPositiveBuckets=DoubleExponentialHistogramBuckets{scale: 20, offset: 1661953, counts: {1661953=4} }, getNegativeBuckets=EmptyExponentialHistogramBuckets{scale=20, offset=0, bucketCounts=[], totalCount=0}, getExemplars=[]}]}}

[dzane17]

Can you add tests?
QueryInsightsListenerTests.testOnRequestEnd should mock searchRequestContext.getHitsLength() and assert the value on the captured record, similar to how isStreamingRequest() / isStreaming() is tested today.
SearchQueryCategorizerTests should verify that recordSearchDocCount is called with the expected value when categorize runs.

[dzane17]

Please add hitsLength to the copy constructor. I know the value is only used for categorization histograms, but we should still keep this method up to date.