Add stats for faiss feature

[jmazanec15]

For the faiss feature, we need to add stats to the stats API. This addition should not break any backwards compatibility. The proposed additional stats will be:

Node Stats

1. training.requests: number of training requests
2. training.errors: number of training request errors
3. training.memory_usage: memory usage used by training in kb
4. training.memory_usage_percentage: memory usage used by training as percentage
5. training.jobs: job level training stats
6. model.indexing_degraded: whether indexing from models is degraded. This is true when the model cache is experiencing capacity misses
7. engine.faiss_in_use: Whether faiss is in use on the node in the last 12 hours
8. engine.nmslib_in_use: Whether nmslib is in use on the node in the last 12 hours

Cluster Stats

1. model.index_status: k-NN model index status

The API response will look like:

{
    "_nodes" : {
        "total" : 1,
        "successful" : 1,
        "failed" : 0
    },
    "cluster_name" : "_run",
    "circuit_breaker_triggered" : false,
    
    "model": {
       "index_status": "green|yellow|red|non-existant",
    },
    
    "nodes" : {
        "HYMrXXsBSamUkcAjhjeN0w" : {
            "cache_capacity_reached" : false,
            "eviction_count" : 0,
            "hit_count" : 0,
            "miss_count" : 1,
            "knn_query_requests" : 4,
            "graph_memory_usage" : 1,
            "graph_memory_usage_percentage" : 3.68,
            "graph_index_requests" : 7,
            "graph_index_errors" : 1,
            "graph_query_requests" : 30,
            "graph_query_errors" : 15,
            "indices_in_cache" : {
                "myindex" : {
                    "graph_memory_usage" : 2,
                    "graph_memory_usage_percentage" : 3.68,
                    "graph_count" : 2
                }
            },
            
            "training": {
                "requests": X,
                "errors": X,
                "memory_usage" : 2,
                "memory_usage_percentage" : 3.68
                "jobs": {
                    "my_model_id" : {
                        "memory_usage" : 2,
                        "memory_usage_percentage" : 3.68
                    }
                },
             },
            "model": {
                "indexing_degraded": true,
            },
            "engine": {
                "nmslib_in_use": false,
                "faiss_in_use": true,
            },

            "load_exception_count" : 0,
            "load_success_count" : 1,
            "total_load_time" : 2878745,
            "script_compilations" : 1,
            "script_compilation_errors" : 0,
            "script_query_requests" : 534,
            "script_query_errors" : 0
        }
    }
}