text: Title: `LOGFIRE_SEND_TO_LOGFIRE` env var doesn't accept `if-token-present` as a value
  Body: ### Description
  
  It would be very useful if it did, also more consistent with `configure()` kwargs
  
  ### Python, Logfire & OS Versions, related packages (not required)
  
  ```TOML
  logfire="0.53.0"
  platform="macOS-14.6.1-arm64-arm-64bit"
  python="3.12.2 (main, Feb 25 2024, 03:55:42) [Clang 17.0.6 ]"
  [related_packages]
  requests="2.32.3"
  pydantic="2.9.2"
  fastapi="0.115.0"
  openai="1.47.1"
  protobuf="4.25.5"
  rich="13.8.1"
  executing="2.1.0"
  opentelemetry-api="1.27.0"
  opentelemetry-exporter-otlp-proto-common="1.27.0"
  opentelemetry-exporter-otlp-proto-http="1.27.0"
  opentelemetry-instrumentation="0.48b0"
  opentelemetry-instrumentation-asgi="0.48b0"
  opentelemetry-instrumentation-asyncpg="0.48b0"
  opentelemetry-instrumentation-fastapi="0.48b0"
  opentelemetry-instrumentation-httpx="0.48b0"
  opentelemetry-instrumentation-system-metrics="0.48b0"
  opentelemetry-proto="1.27.0"
  opentelemetry-sdk="1.27.0"
  opentelemetry-semantic-conventions="0.48b0"
  opentelemetry-util-http="0.48b0"
  ```
  