access_log: refactored SubstitutionFormatParser::parseCommand

[cpakulski]

Commit Message:
refactored SubstitutionFormatParser::parseCommand

Additional Description:
This PR addresses few items:

• tech debt: there was a TODO comment that parsing access_log command should use string_view to avoid copying. After using string_view SubstitutionFormatParser::parseCommand method is around 40% faster. Microbenchmark reports the following results:

Benchmark Time CPU Iterations
BM_FormatterCommandParsingOld 2194 ns 2194 ns 319163
BM_FormatterCommandParsingNew 1260 ns 1260 ns 554762

• SubstitutionFormatParser::parseCommand has been implemented using variadic template. This is done in preparation for issue access_log: support for logging route/listener/cluster metadata #3006. The idea has been proposed on envoy-dev slack on April 14th 2021. In a nutshell, there will be one tag for all metadata in form METADATA(TYPE:NAMESPACE:KEY):Z. Notice that there is additional tag TYPE. Using Variadic template for parsing commands will automatically address the presence of extra tag.

Risk Level: Low
Testing: Added new unit tests and microbenchmark
Docs Changes: Not required
Release Notes: Not required

Platform Specific Features:
[Optional Runtime guard:]
[Optional Fixes #Issue]
[Optional Deprecated:]
[Optional API Considerations:]

[cpakulski]

/retest

[repokitteh-read-only]

Retrying Azure Pipelines:
Check envoy-presubmit isn't fully completed, but will still attempt retrying.
Retried failed jobs in: envoy-presubmit

🐱

Caused by: a #16121 (comment) was created by @cpakulski.

see: more, trace.

[cpakulski]

/retest

[repokitteh-read-only]

Retrying Azure Pipelines:
Retried failed jobs in: envoy-presubmit

🐱

Caused by: a #16121 (comment) was created by @cpakulski.

see: more, trace.

[yanavlasov]

Thanks, this looks really good. Could you add some negative tests as well, i.e. when command is malformed to test parsing error handling, please?

[yanavlasov]

/wait

[cpakulski]

@yanavlasov Thanks for reviewing and merging.

[rgs1]

FYI this broke our internal formatter extensions:

16:48:27 pinterest/formatters/dynamic_metadata_unquoted/formatter.cc:54:5: error: no matching function for call to 'parseCommand'
16:48:27     Envoy::Formatter::SubstitutionFormatParser::parseCommand(token, start, ":", filter_namespace,
16:48:27     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Not a huge deal, but something to take in account code that might be relied upon by extensions...

[cpakulski]

@rgs1 I apologize for that. I could probably leave original methods with original signatures and map it to the new ones.
I am guessing that the problem is with different order of parameters.

[rgs1]

@cpakulski no worries at all, the change was easy. Mostly just posted for visibility, since eventually formatter extensions might become more popular (I suspect they are not heavily used right now).