semconv: Add metric generation

[carrbs]

Closes: #4528

This is a first attempt in generating some metric constants as a first step in completing deeper generation of semantic conventions for metrics. I borrowed a bit of the jinja logic from opentelemetry-python, as well as some of the formatting functions from the jinja template used in this repos semconv directory.

After taking a look at the RELEASE.md, I'm now thinking the generated semconv/v1.24.0/metric.go file shouldn't be included in this PR.

[carrbs]

I added some more consts in this last commit. This would allow us to change the usage mentioned in #4528 like this:

opentelemetry-go/semconv/v1.24.0/metric.go

Lines 729 to 737 in bd60b70

	// RPCServerDuration is the metric conforming to the "rpc.server.duration"
	// semantic conventions. It represents the measures the duration of inbound
	// RPC.
	// Instrument: histogram
	// Unit: ms
	// Stability: None
	RPCServerDurationName = attribute.Key("rpc.server.duration")
	RPCServerDurationUnit = attribute.Key("ms")
	RPCServerDurationDescription = attribute.Key("Measures the duration of inbound RPC.")

from opentelemetry-go-contrib/blob/main/instrumentation/google.golang.org/grpc/otelgrpc/config.go:

// original
c.rpcDuration, err = c.meter.Float64Histogram("rpc."+role+".duration",
	metric.WithDescription("Measures the duration of inbound RPC."),
	metric.WithUnit("ms"))

// with generated consts
c.rpcDuration, err = c.meter.Float64Histogram(semconv.RPCServerDurationName,
	metric.WithDescription(semconv.RPCServerDurationDescription),
	metric.WithUnit(semconv.RPCServerDurationUnit))

@pellared suggested "at least the instrument name [could be generated], but maybe something that creates a whole instrument would be better? but it may be too complex". I think it's likely possible we can gather these items together, possibly in the jinja. Shouldn't be too complex. I want to get some feedback on the current implementation to make sure it's on track.

A few other notes for discussing:

• I noticed that the usage case here hardcodes "Measures the duration of inbound RPC" even though the role is a variable. (i.e., the message should be "Measures the duration of outbound RPC" when role == 'client').
• I came across metric_group as an option for identifying a specific type of semantic convention, but I don't see any definitions (outside of some test data in the build_tools repo) utilizing it. Likely a question for semantic conventions SIG group.
• many of the metrics have attributes, and many of those attributes have refs. I'm not sure if those should be considered as scope for this. Ex:

// SystemCPUTime is the metric conforming to the "system.cpu.time" semantic
	// conventions. It represents the seconds each logical CPU spent on each mode
	// Instrument: counter
	// Unit: s
	// Stability: None

	// attribute: system.cpu.logical_number

	// attribute: system.cpu.state

	SystemCPUTimeName        = attribute.Key("system.cpu.time")
	SystemCPUTimeUnit        = attribute.Key("s")
	SystemCPUTimeDescription = attribute.Key("Seconds each logical CPU spent on each mode")

[carrbs]

Examples of generated code and comments:

• defined stability and brief (notice the it_reps() output can be duplicative, adding "It represents..." to all briefs:

	// RPCServerDuration is the metric conforming to the "rpc.server.duration"
	// semantic conventions. It represents the measures the duration of inbound
	// RPC.
	// Instrument: histogram
	// Unit: ms
	// Stability: Experimental
	RPCServerDurationName        = "rpc.server.duration"
	RPCServerDurationUnit        = "ms"
	RPCServerDurationDescription = "Measures the duration of inbound RPC."

• when undefined stability/brief in semantic-conventions:

	// SystemFilesystemUsage is the metric conforming to the
	// "system.filesystem.usage" semantic conventions.
	// Instrument: updowncounter
	// Unit: By
	// Stability: Experimental
	// NOTE: The description (brief) for this metric is not defined in the semantic-conventions repository.
	SystemFilesystemUsageName = "system.filesystem.usage"
	SystemFilesystemUsageUnit = "By"

[pellared]

I found some capitalization issues.

Overall it looks very nice 👍

[carrbs]

Ticket for adding the custom capitalizations flag to semconvgen: open-telemetry/opentelemetry-go-build-tools#528

[carrbs]

PR (WIP) for adding the flag and processing of custom capitalizations.

[pellared]

@open-telemetry/go-approvers PTAL

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 83.7%. Comparing base (35c9570) to head (04f11d0).

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #4880     +/-   ##
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- Coverage   83.7%   83.7%   -0.1%     
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  Files        252     252             
  Lines      16458   16458             
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- Hits       13780   13778      -2     
- Misses      2387    2389      +2     
  Partials     291     291             

see 1 file with indirect coverage changes