logging: add option to use android logger for process logs

[junr03]

Description: this PR adds an option to select different process loggers based on bazel config. Notably it introduces an Android Logger that is able to log to Android App's log stream.
Risk Level: low. The new functionality is bazel config guarded.
Testing: local. CI
Docs Changes: documented in bazel's README.md

[junr03]

@jmarantz do you mind taking a look at this approach?

My initial thought was to derive from the SinkDelegate class structure. However, __android_log_write needs level information, and the android log levels are different from the spdlog and envoy log levels. Conveniently, spdlog already has an android sink, so I leveraged that in this PR.

I went for the lowest level at which I could interact with a spdlog::sinks::sink, and that was replacing the DelegatingLogSink whole sale when we construct Logger's logger_. This seemed cleaned enough, but do let me know if you can think of something else. Very open to suggestions.

[junr03]

@zuercher do you mind taking a look?

[zuercher]

I think this is generally fine, but I had a question about the bazel side of this.

[zuercher]

Given that the andoid sink in spdlog is #ifdef __ANDROID__, does it make sense to gate this logger on the android equivalent of //bazel::windows_x86_64 and //bazel:apple (instead of a flag that won't work in any other env) and following the pattern used for envoy_cc_win32_library?

(The file watcher lib should get the same treatment for apple, but that's another PR.)

[junr03]

That was my first idea, but @goaway pointed out that there might be cases where we might want the StandardLogger when __ANDROID__ is defined, e.g running envoy as a process in an android device, rather than in a sandboxed application.

[zuercher]

Ok. Do we want to pull the selection logic out into helpers?

[junr03]

I also debated about this. Given that it was only used once now and for the foreseeable future I opted for not introducing more helpers into the build system until they are needed. But if you think it is worth it, I am happy to add!

[zuercher]

I think the thing I'm getting it is whether having an empty envoy_cc_library (logger_impl_lib_android) when android logging is disabled causes some kind of problem for bazel (particular the query subcommand). It seemed like there was something related to that with win32 libs, but I'm a bit out of my depth when it comes to bazel. I suppose it can always be fixed later.

[junr03]

I see. I was copying the pattern from win32

envoy/bazel/envoy_library.bzl

Line 151 in b78fc4e

def envoy_cc_win32_library(name, srcs = [], hdrs = [], **kargs):

. Yeah, I agree, if there is a reported problem we can fix later.