The Grafana data source plugin for Instana is designed to query and retrieve metric data directly from the Instana backend. This communication is facilitated through the Instana Web REST API. By definition, a REST (Representational State Transfer) API operates over the HTTP (Hypertext Transfer Protocol) or HTTPS protocol. The Grafana plugin acts as an HTTP client, sending requests to the Instana API endpoints to fetch the necessary data for visualization in Grafana dashboards.

A. gRPC: This is a high-performance RPC framework. The Instana Web REST API, which the Grafana plugin uses, is based on HTTP, not gRPC.

B. SOP: Same-Origin Policy is a web browser security mechanism that restricts cross-origin HTTP requests, not a protocol for fetching data.

D. JDBC: Java Database Connectivity (JDBC) is an API for connecting Java applications to databases. The Grafana plugin connects to the Instana application API, not directly to a database via JDBC.

1. IBM Documentation - Grafana plugin: "The Instana data source plugin for Grafana uses our Web REST API to fetch infrastructure metrics from Instana and to make them available in your Grafana dashboards." (Source: IBM Documentation, Instana Observability, Section: Integrating > Dashboards > Grafana plugin).

2. IBM Documentation - Web REST API: This section details the various HTTP-based endpoints available for interacting with Instana, which is the foundation for the Grafana plugin's data retrieval mechanism. (Source: IBM Documentation, Instana Observability, Section: Developing > Web REST API).