Function to_grpc_insert_requests 

pub fn to_grpc_insert_requests(
    request: ExportMetricsServiceRequest,
    metric_ctx: &mut OtlpMetricCtx,
) -> Result<(RowInsertRequests, usize, SemanticIndex)>

Convert OpenTelemetry metrics to GreptimeDB insert requests

See https://github.com/open-telemetry/opentelemetry-proto/blob/main/opentelemetry/proto/metrics/v1/metrics.proto for data structure of OTLP metrics.

Returns InsertRequests, total number of rows to ingest, and the per-table semantic index for the auto-create path to stamp as table options.