Make member function as const whenever possible.

Test: unit tests passed.
Change-Id: I751cabf305a4b5aa2095853cc951837da0df4c78
diff --git a/cmds/statsd/src/StatsLogProcessor.cpp b/cmds/statsd/src/StatsLogProcessor.cpp
index fcb5107..01a58d5 100644
--- a/cmds/statsd/src/StatsLogProcessor.cpp
+++ b/cmds/statsd/src/StatsLogProcessor.cpp
@@ -114,7 +114,7 @@
     }
 }
 
-size_t StatsLogProcessor::GetMetricsSize(const ConfigKey& key) {
+size_t StatsLogProcessor::GetMetricsSize(const ConfigKey& key) const {
     auto it = mMetricsManagers.find(key);
     if (it == mMetricsManagers.end()) {
         ALOGW("Config source %s does not exist", key.ToString().c_str());