Collect telemetry for semantic_text usage

[ioanatia]

this is a draft, this is not finalized by any means.
among other things, we need to settle on the format of the payload first, but wanted to show what's possible to track right now.

example of payload:

    "semantic_text": {
      "total_fields": 5, // total number of semantic_text fields in all indices
      "indices": 4, // number of indices that have at least one semantic_text field
      "dense_fields": 0, // how many of them use dense embeddings
      "dense_inference_ids": 0, // how many text_embedding inference IDs are used by semantic_text fields
      "sparse_fields": 5, // how many semantic_text fields use sparse embeddings
      "sparse_inference_ids": 1 // how many sparse_embedding inference IDs are used by semantic_text fields
    }

the one thing I have not added is tracking what services are for the inference endpoints, but it should be completely doable since we have that informantion in ModelConfigurations.

[ioanatia]

going to close this since @dimitris-athanasiou is going to look into it