refactor: Introduce FilesystemStatistics for adding runtime counters (#…

…145)

Summary:
X-link: facebookincubator/velox#12424


Introduce FilesystemStatistics as a free form thread-safe structure used to create populate counters based off of Velox's RuntimeMetrics. The underlying data structure is a map with key being whatever client would like and value being the RuntimeMetrics. 

The motivation behind this refactor is that previous PR introduced iostats into filesystems.h interface; however the iostats is specific to dwio and potentially not generic for other filesystem implementations. This PR addresses this by not using iostats and introducing `FilesystemStatistics` to serve the same purpose and also allow generic free form stats for user to add.

Differential Revision: D70007870

dwio/nimble/common/tests/TestUtils.h

@@ -294,30 +294,31 @@ class InMemoryTrackableReadFile final : public velox::ReadFile {
       uint64_t offset,
       uint64_t length,
       void* buf,
-      velox::io::IoStatistics* stats = nullptr) const final {
+      velox::filesystems::FileSystemStatistics* stats = nullptr) const final {
     chunks_.wlock()->push_back({offset, length});
     return file_.pread(offset, length, buf, stats);
   }
 
   std::string pread(
       uint64_t offset,
       uint64_t length,
-      velox::io::IoStatistics* stats = nullptr) const final {
+      velox::filesystems::FileSystemStatistics* stats = nullptr) const final {
     chunks_.wlock()->push_back({offset, length});
     return file_.pread(offset, length, stats);
   }
 
   uint64_t preadv(
       uint64_t /* offset */,
       const std::vector<folly::Range<char*>>& /* buffers */,
-      velox::io::IoStatistics* stats = nullptr) const final {
+      velox::filesystems::FileSystemStatistics* stats = nullptr) const final {
     NIMBLE_NOT_SUPPORTED("Not used by Nimble");
   }
 
   uint64_t preadv(
       folly::Range<const velox::common::Region*> regions,
       folly::Range<folly::IOBuf*> iobufs,
-      velox::io::IoStatistics* stats = nullptr) const override {
+      velox::filesystems::FileSystemStatistics* stats =
+          nullptr) const override {
     VELOX_CHECK_EQ(regions.size(), iobufs.size());
     uint64_t length = 0;
     for (size_t i = 0; i < regions.size(); ++i) {