bump prometheus dependency to 0.13

0.13.0

Bug fix: Avoid panics from Instant::elapsed (open-telemetry#406)

Improvement: Allow trailing comma on macros (open-telemetry#390)

Improvement: Add macros for custom registry (open-telemetry#396)

Improvement: Export thread count from process_collector (open-telemetry#401)

Improvement: Add convenience TextEncoder functions to encode directly to string (open-telemetry#402)

Internal change: Clean up the use of macro_use and extern crate (open-telemetry#398)

Internal change: Update dependencies

opentelemetry-prometheus/Cargo.toml

@@ -21,7 +21,7 @@ rustdoc-args = ["--cfg", "docsrs"]
 
 [dependencies]
 opentelemetry = { version = "0.16", path = "../opentelemetry", default-features = false, features = ["metrics"] }
-prometheus = "0.12"
+prometheus = "0.13"
 protobuf = "2.14"
 
 [dev-dependencies]