feat: Expose ledger cache full and disabled to prometheus (#1957)

Fixes #1771

tests/unit/CMakeLists.txt

@@ -11,6 +11,7 @@ target_sources(
           data/AmendmentCenterTests.cpp
           data/BackendCountersTests.cpp
           data/BackendInterfaceTests.cpp
+          data/LedgerCacheTests.cpp
           data/cassandra/AsyncExecutorTests.cpp
           data/cassandra/ExecutionStrategyTests.cpp
           data/cassandra/RetryPolicyTests.cpp

tests/unit/data/LedgerCacheTests.cpp

@@ -0,0 +1,68 @@
+//------------------------------------------------------------------------------
+/*
+    This file is part of clio: https://github.com/XRPLF/clio
+    Copyright (c) 2025, the clio developers.
+
+    Permission to use, copy, modify, and distribute this software for any
+    purpose with or without fee is hereby granted, provided that the above
+    copyright notice and this permission notice appear in all copies.
+
+    THE  SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
+    WITH  REGARD  TO  THIS  SOFTWARE  INCLUDING  ALL  IMPLIED  WARRANTIES  OF
+    MERCHANTABILITY  AND  FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
+    ANY  SPECIAL,  DIRECT,  INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
+    WHATSOEVER  RESULTING  FROM  LOSS  OF USE, DATA OR PROFITS, WHETHER IN AN
+    ACTION  OF  CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
+    OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
+*/
+//==============================================================================
+
+#include "data/LedgerCache.hpp"
+#include "util/LoggerFixtures.hpp"
+#include "util/MockPrometheus.hpp"
+#include "util/prometheus/Bool.hpp"
+
+#include <gmock/gmock.h>
+#include <gtest/gtest.h>
+
+using namespace data;
+
+struct LedgerCacheTest : util::prometheus::WithPrometheus, NoLoggerFixture {
+    LedgerCache cache;
+};
+
+TEST_F(LedgerCacheTest, defaultState)
+{
+    EXPECT_FALSE(cache.isDisabled());
+    EXPECT_FALSE(cache.isFull());
+    EXPECT_EQ(cache.size(), 0u);
+    EXPECT_EQ(cache.latestLedgerSequence(), 0u);
+}
+
+struct LedgerCachePrometheusMetricTest : util::prometheus::WithMockPrometheus, NoLoggerFixture {
+    LedgerCache cache;
+};
+
+TEST_F(LedgerCachePrometheusMetricTest, setDisabled)
+{
+    auto& disabledMock = makeMock<util::prometheus::Bool>("ledger_cache_disabled", {});
+
+    EXPECT_CALL(disabledMock, set(1));
+    cache.setDisabled();
+
+    EXPECT_CALL(disabledMock, value()).WillOnce(testing::Return(1));
+    EXPECT_TRUE(cache.isDisabled());
+}
+
+TEST_F(LedgerCachePrometheusMetricTest, setFull)
+{
+    auto& fullMock = makeMock<util::prometheus::Bool>("ledger_cache_full", {});
+    auto& disabledMock = makeMock<util::prometheus::Bool>("ledger_cache_disabled", {});
+
+    EXPECT_CALL(disabledMock, value()).WillOnce(testing::Return(0));
+    EXPECT_CALL(fullMock, set(1));
+    cache.setFull();
+
+    EXPECT_CALL(fullMock, value()).WillOnce(testing::Return(1));
+    EXPECT_TRUE(cache.isFull());
+}