access log: added new COMMON_DURATION command

[wbpcode]

Commit Message: access log: added new COMMON_DURATION command
Additional Description:

Added :ref:%COMMON_DURATION% to access log format to log the duration of the request. The start time point, end time point and duration precision can be configured in this new command.
For example, given following command
%COMMON_DURATION(DS_RX_BEG, US_TX_BEG, us)%,
Envoy will log the duration from the downstream request start to the upstream request start in the microseconds.

To close #33001

Risk Level: low.
Testing: unit.
Docs Changes: added.
Release Notes: added.
Platform Specific Features: n/a.

[wbpcode]

I personally think the name of the time point is too long, may be we should use some short names here?

Here are current supported time point name:

    * ``FIRST_DOWNSTREAM_RX_BYTE_RECEIVED``: The time point of the first byte of downstream request is received. Same as ``START``.
    * ``LAST_DOWNSTREAM_RX_BYTE_RECEIVED``: The time point of the last byte of downstream request is received.
    * ``FIRST_UPSTREAM_TX_BYTE_SENT``: The time point of the first byte of upstream request is sent.
    * ``LAST_UPSTREAM_TX_BYTE_SENT``: The time point of the last byte of upstream request is sent.
    * ``FIRST_UPSTREAM_RX_BYTE_RECEIVED``: The time point of the first byte of upstream response is received.
    * ``LAST_UPSTREAM_RX_BYTE_RECEIVED``: The time point of the last byte of upstream response is received.
    * ``FIRST_DOWNSTREAM_TX_BYTE_SENT``: The time point of the first byte of downstream response is sent.
    * ``LAST_DOWNSTREAM_TX_BYTE_SENT``: The time point of the last byte of downstream response is sent.
    * Dynamic value: Any other values will be treated as custom time points that set by named keys in the downstream timing.

May be we can use some short abbreviations, for example:

DS_RX_BEG：FIRST_DOWNSTREAM_RX_BYTE_RECEIVED
US_TX_END：LAST_UPSTREAM_TX_BYTE_SENT

Then, the
%COMMON_DURATION(FIRST_DOWNSTREAM_RX_BYTE_RECEIVED, LAST_UPSTREAM_TX_BYTE_SENT)%
will become the
%COMMON_DURATION(DS_RX_BEG, US_TX_END)%
which is much shorter.

[wbpcode]

This is new feature, so let's use the short name. it will make the log template simpler

    * ``DS_RX_BEG``: The time point of the downstream request receiving begin.
    * ``DS_RX_END``: The time point of the downstream request receiving end.
    * ``US_TX_BEG``: The time point of the upstream request sending begin.
    * ``US_TX_END``: The time point of the upstream request sending end.
    * ``US_RX_BEG``: The time point of the upstream response receiving begin.
    * ``US_RX_END``: The time point of the upstream response receiving end.
    * ``DS_TX_BEG``: The time point of the downstream response sending begin.
    * ``DS_TX_END``: The time point of the downstream response sending end.
    * Dynamic value: Other values will be treated as custom time points that set by named keys.

    The PRECISION is specified by the following values:

    * ``ms``: Millisecond precision.
    * ``us``: Microsecond precision.
    * ``ns``: Nanosecond precision.

[wbpcode]

/retest

[zirain]

this will be really helpful when debuging, thanks!

[zuercher]

Generally, looks good. Some minor comments below.

[wbpcode]

/retest