Add UsageTableHeader metrics

[ Merge from http://go/wvgerrit/72183 ]

Adds basic metrics for the usage table header.

Bug: http://b/112919252
Test: Unit tests. Manual GPlay
Change-Id: I14d406766d8b2aba3b6e4f1a02c75eedc674b011

libwvdrmengine/cdm/core/test/cdm_session_unittest.cpp

@@ -361,7 +361,7 @@ TEST_F(CdmSessionTest, UpdateUsageEntry) {
   std::string serialized_metrics;
   ASSERT_TRUE(metrics.SerializeToString(&serialized_metrics));
   EXPECT_GT(metrics.crypto_metrics()
-            .crypto_session_update_usage_entry_time_us().size(), 0)
+            .usage_table_header_update_entry_time_us().size(), 0)
       << "Missing update usage entry metric. Metrics: "
       << wvcdm::b2a_hex(serialized_metrics);
 }