[Filestore] implement availability counters for FUSE requests (#6826)

### Notes

A Shared Filesystem is considered **unavailable** during any `N-minute`
interval in which, for at least one filesystem request type, at least
one request was **outstanding** and every such request either failed
with an **EIO** error, or **hung**.

**Note**: A request submitted during an interval and still outstanding
when the interval ends is not classified for that interval. It does not
contribute to determining whether the interval is available or
unavailable

In all `N-minute` intervals in which the unavailability condition is not
met, a Shared Filesystem is considered **available**.

Shared Filesystem availability for the measurement period shall be
calculated as:
Number of `N-minute` intervals in which the Shared Filesystem was
Available / Total number of `N-minute` intervals in the measurement
period × 100%

### Definitions

A request is **outstanding** at a given point in time if it was
submitted but had received no response, success or error, by that point.
A request is **hung** if it was **outstanding** for the entire duration
of the `N-minute` interval.
Request type - a category of filesystem operation, Fuse request types:
```
readdir
readdirplus
opendir
releasedir
lookup
getattr
setattr
write
write_buf
read
open
create
release
mkdir
rmdir
unlink
rename
link
symlink
readlink
flush
fsync
```

EIO error - POSIX EIO error code (see
https://man7.org/linux/man-pages/man3/errno.3.html)

### Issue
#6825

Author: SvartMetal

cloud/filestore/config/filesystem.proto

@@ -127,4 +127,10 @@ message TFileSystemConfig
 
     // Async processing of read-only create handle requests.
     optional bool AsyncCreateHandleEnabled = 37;
+
+    // Enables the per-client filesystem availability tracking.
+    optional bool AvailabilityTrackingEnabled = 38;
+
+    // Availability interval duration (in ms); 0 means the default.
+    optional uint32 AvailabilityTrackingInterval = 39;
 }

cloud/filestore/config/storage.proto

@@ -896,4 +896,10 @@ message TStorageConfig
 
     // Sets the time after which latency of a node + request tuple will decrease by half.
     optional uint32 NodeLatencyHalfLife = 538; // measured in ms
+
+    // Enables the per-client filesystem availability tracking on the guest.
+    optional bool AvailabilityTrackingEnabled = 539;
+
+    // Availability interval duration (in ms); 0 means the default.
+    optional uint32 AvailabilityTrackingInterval = 540;
 }