Add metrics for raft message send wait & receive delay duration

## Send wait duration

This phase begins when the RaftStore thread sends the RaftMessage
to the RaftClient and ends when the RaftMessage leaves the
BatchRaftMessage buffer, just before being flushed to the gRPC client.
Since this phase occurs entirely within the same node, we measure it
using monotonic time.

## Receive delay duration

Receive delay duration can also be called send duration. The name 
'Receive delay duration' is used because this duration is reported 
by the receiver, making it more clearer.

This phase begins after the send wait ends and continues as the message
is sent over the network, ends when the target peer receives it.
Since this phase spans across nodes, we measure it using physical time.
To facilitate this, we’ve introduced a last_observed_time field in
the BatchRaftMessage to record the physical times (please also review
the related PR: https://github.com/pingcap/kvproto/pull/1276).
Although physical clock drift between nodes is a possibility, its
impact on our measurement is limited. Our primary goal is to observe
trends in duration changes rather than relying on absolute precision.

NOTE: Metrics are only added for the batch_raft RPC, as the raft
RPC is deprecated and no longer tracked.

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