Update URL that grabs tar.gz for prometheus client_model (#294)

Signed-off-by: rulex123 <[email protected]>

Author: rulex123

tools/update-api.sh

@@ -50,7 +50,7 @@ curl -sL https://github.com/census-instrumentation/opencensus-proto/archive/v${O
 mkdir -p "${protodir}/opencensus/proto"
 cp -r opencensus-proto-*/src/opencensus/proto/* "${protodir}/opencensus/proto"
 
-curl -sL https://github.com/prometheus/client_model/archive/${PROMETHEUS_SHA}.tar.gz | tar xz --wildcards '*.proto'
+curl -sL https://github.com/prometheus/client_model/archive/v${PROMETHEUS_SHA}.tar.gz | tar xz --wildcards '*.proto'
 mkdir -p "${protodir}/io/prometheus/client/"
 cp client_model-*/io/prometheus/client/metrics.proto "${protodir}/io/prometheus/client/"