Bump version to 0.3.1 in Project.toml (#122)

Author: findmyway

src/exporter/prometheus/Project.toml

@@ -1,7 +1,7 @@
 name = "OpenTelemetryExporterPrometheus"
 uuid = "34e26579-e93c-4b5e-ba07-7afb633408c2"
 authors = ["Jun Tian <tianjun.cpp@gmail.com>"]
-version = "0.3.0"
+version = "0.3.1"
 
 [deps]
 Base64 = "2a0f44e3-6c83-55bd-87e4-b1978d98bd5f"