parse configured EnvFilter once

This allows the error handling to be done upfront instead of for each
use. In particular, the `toml` error now points to the span of text in
the config file where the misconfigured EnvFilter value is. This is much
better than the previous error that did not indicate what was actually
causing it to happen.

src/error.rs

@@ -61,9 +61,6 @@ pub(crate) enum Observability {
     #[error("opentelemetry error")]
     Otel(#[from] opentelemetry::trace::TraceError),
 
-    #[error("invalid log filter syntax")]
-    EnvFilter(#[from] tracing_subscriber::filter::ParseError),
-
     #[error("failed to install global default tracing subscriber")]
     SetSubscriber(#[from] tracing::subscriber::SetGlobalDefaultError),