Emit a searchLatency metric on the OpenSearch source

Is your feature request related to a problem? Please describe.
As a user of the OpenSearch source, it is not clear to me how long the search requests are taking to be fulfilled by OpenSearch.

Describe the solution you'd like
A searchLatency metric that provides information on how long it takes for the actual search request to take place. This metric may not be that necessary as OpenSearch commonly provides a searchLatency metric, but this would identify the latency specifically coming from Data Prepper searches

There is already an indexProcessingTime metric that tracks how long particular indices take to process, but not one that shows the search requests themselves to identify if OpenSearch is a bottleneck in the migration.

Start in the OpenSearch source by locating the existing indexProcessingTime metric and the code that performs search requests. Trace where a search begins and completes, then inspect nearby metric tests if present. Done means a searchLatency metric reports the time spent on the actual OpenSearch search request and is covered by tests.